Convert Images to WebP, JPEG or PNG and Hit Any File Size — Free, In Your Browser
There's a specific frustration that comes with uploading an image to WordPress and watching your PageSpeed score drop because the file is 1.2 MB. Or realising the PNG you exported from Figma is 800KB when the Joomla template expects images under 150KB. The image looks the same either way — the problem is purely the file.
This article covers which format to use for which situation, what file sizes actually matter for technical SEO and Core Web Vitals, and how to convert and compress images to hit a specific KB target without uploading them anywhere.
JPEG, WebP, PNG — which one and when
The format question comes up constantly and there's usually one right answer per use case. Here's what that looks like in practice:
Best for most web images
25–35% smaller than JPEG at equivalent quality. Supported in all modern browsers. WordPress supports it natively since version 5.8 (2021). Use this for blog images, product photos, hero banners, anything that needs to load fast.
Best for photos going outside the web
Universal compatibility — email clients, older apps, print. Use JPEG when the image is going somewhere that might not support WebP. Also fine for WordPress if you're not ready to switch formats across the board.
Best for transparency and sharp edges
Logos, icons, screenshots with text, anything with a transparent background. PNG files are larger than WebP or JPEG for photos, so don't use PNG for photography. Use it when you need pixel-perfect edges or transparency.
The short version: convert your photos and blog images to WebP, keep logos as PNG, use JPEG for anything that needs to work outside a modern browser.
Why file size matters more than most people realise
Google's Core Web Vitals measure three things. Largest Contentful Paint (LCP) is the one that images directly control — it measures how long it takes for the biggest visible element on the page to load. For most pages, that element is an image.
Google's own data shows that LCP above 2.5 seconds is considered "needs improvement," and images are the most common cause of slow LCP scores. A 1MB hero image on a mobile connection can push LCP to 4-5 seconds even on a good 4G signal.
Beyond Core Web Vitals, image file size affects:
- Crawl budget — search bots have a time limit per site. Pages that load slowly eat into that budget.
- Mobile rankings — Google uses mobile-first indexing. Mobile users on slower connections are the benchmark.
- Hosting bandwidth — smaller files mean lower storage and transfer costs, especially relevant on shared hosting plans used by most WordPress sites.
The file size targets worth using for WordPress and Joomla
These aren't hard rules but they're a solid starting point for most CMS sites:
| Image type | Recommended max size | Best format |
|---|---|---|
| Hero / banner (full width) | 150–200 KB | WebP |
| Blog post feature image | 80–120 KB | WebP |
| Product image (e-commerce) | 100–150 KB | WebP or JPEG |
| Thumbnail / card image | 30–60 KB | WebP |
| Logo | Under 20 KB | PNG or SVG |
| Icon / small UI element | Under 10 KB | PNG or SVG |
The tricky part is that "compress to 100KB" isn't a setting in most tools. They give you a quality slider (0–100%) and you have to guess what that maps to in KB, re-export, check the file size, adjust, and try again. For one image that's annoying. For a batch of 20 product photos it's genuinely painful.
What most online converters get wrong
The usual free converters online have one of three problems. They upload your images to their servers — which is a concern for anything not yet published, for client work, or for proprietary product photos. They have file size limits on the free tier (typically 5–10MB per file). Or they don't have a target KB feature, only a quality percentage.
Why uploading matters: when you upload an image to a web converter, you're giving a third-party service a copy of your file. For most blog photos this is fine. For client deliverables, unreleased product photography, or anything with a confidentiality expectation, it's worth thinking about before you click upload.
How the tools compare
| Tool | Target KB | WebP output | No upload | Batch |
|---|---|---|---|---|
| Squoosh (Google) | ✗ Quality % only | ✓ | ✓ Local | ✗ One file at a time |
| TinyPNG / TinyJPG | ✗ No target KB | ⚬ Paid (TinyWand) | ✗ Uploads to server | ⚬ 20 files free |
| iLoveIMG | ✗ Quality % only | ✓ | ✗ Uploads to server | ⚬ Limited free |
| Canva (image resize) | ✗ No target KB | ✗ JPEG/PNG only | ✗ Uploads to Canva | ✗ One at a time |
| Free Auto Captions AI Image Studio | ✓ Set exact KB target | ✓ JPEG, PNG, WebP | ✓ Runs in browser | ✓ Batch support |
How to convert and compress your images in the browser
Free Auto Captions AI's Image Studio handles format conversion and target file size compression entirely in your browser. Your image files stay on your device throughout.
Open Free Auto Captions AI and go to Image Studio
No model download needed for image tools — they load instantly. You can drag a single file or drop a whole batch.
Choose your output format
Select WebP, JPEG, or PNG. For WordPress blog images, WebP is almost always the right choice. If you're exporting for an email newsletter or external platform, stick with JPEG.
Set your target file size in KB
Enter the KB target you want — 100KB for a blog image, 150KB for a hero banner, 50KB for a thumbnail. The tool compresses to fit within that limit while keeping as much quality as possible.
Download and upload to your CMS
Download the converted files and upload them to WordPress, Joomla, Webflow, or wherever they're going. The file names stay the same so you can swap them into existing pages without changing references.
Batch tip: if you have a folder of mixed JPEG and PNG images you want to standardise as WebP under 100KB, drop them all in at once. The tool processes each one independently to hit the target, so a simple 30KB icon and a complex 2MB photo both end up under 100KB without you having to adjust each manually.
WordPress-specific tips
WordPress handles image uploads through the media library, and there are a few things worth knowing:
WordPress generates multiple image sizes automatically (thumbnail, medium, large, full) whenever you upload. If you upload a 2MB PNG, those auto-generated sizes are also large PNGs. If you upload a 90KB WebP, the auto-generated sizes inherit the smaller format. It's much better to get the source file right before uploading than to rely on WordPress's built-in compression.
Plugins like Smush or ShortPixel can bulk-convert your existing media library to WebP after the fact, but they usually have free-tier limits and still generate server load. Pre-converting before upload avoids the plugin dependency entirely.
If your Joomla site uses a page builder or template with fixed image dimensions, check what pixel dimensions it expects. An image that's 4000px wide will never load fast even if the file size is compressed — scale the dimensions down in Image Studio alongside the file size.
The impact on technical SEO
Getting images right has a measurable effect on rankings, but mainly indirectly. The direct signals are Core Web Vitals (which Google has confirmed as a ranking factor since 2021) and page speed. The indirect effects are engagement: pages that load in under 2 seconds have lower bounce rates, and lower bounce rates correlate with better rankings over time.
For a typical WordPress blog post, switching feature images from 500KB JPEGs to 90KB WebPs can cut page weight by 30-50% and push LCP from the "needs improvement" zone into the "good" range. That's not a minor SEO win — it's one of the highest-leverage technical changes available on most content sites.
Convert and compress your images — nothing leaves your device
JPEG, WebP, PNG. Set a target KB. Batch process. All in your browser.
🖼️ Open Image Studio