Every "free online converter" asks you to upload the file first. For a passport scan, a client mockup, or a photo of your ID, that is a strange thing to do — you are handing a stranger's server a copy of the file and hoping it gets deleted.
OnliTool's Image Converter takes the opposite route: the file never leaves the tab. It decodes the image with the browser, re-encodes it with the browser's own encoder, and hands you a download. No account, no queue, no server-side copy.
Here is what you can actually do with it, then how it behaves when you push real files through.
Five situations where this earns its place
1. A portal only accepts JPG, and your phone shoots HEIC.
Job applications, visa forms, insurance claims and school portals are still built around .jpg. An iPhone writes .heic. The converter reads HEIC/HEIF and writes JPG, PNG or WebP, so you can satisfy the form without installing anything or emailing the file to yourself.
2. A design site handed you AVIF or WebP, and PowerPoint refuses it.
Modern stock sites and CDNs serve .avif and .webp. Word, PowerPoint and plenty of desktop apps still do not open them. Drop the file in, pick PNG (or JPG), done.
3. Your blog screenshots are PNG and weigh too much.
A screenshot of a UI is mostly flat colour and text, which is exactly where WebP wins. In my test a 236 KB PNG screenshot came out at 57.8 KB as WebP — same pixels, a quarter of the bytes.
4. You have a whole folder to fix, not one file.
Product shots, scanned paperwork, a folder of client logos. Select them all at once; every file gets a row, one click converts the batch, and "Download all" gives you a single ZIP with the original names and the new extension.
5. You just need to see whether the other side can open it.
Sometimes the goal is not size, it is compatibility: turning an exotic format into the boring one the recipient's machine understands. That is a two-second job here, and nothing is uploaded while you check.
What the tool is
Three properties matter more than the feature list:
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Batch input. The file picker accepts multiple files and
image/*plus.heic,.heif,.avif. JPG, PNG, WebP, AVIF, GIF, BMP, SVG and HEIC all come in. - Four outputs. JPG, PNG, WebP, AVIF. That is the whole settings panel — no quality slider, no resize field.
-
Local execution. Decoding and encoding happen in the page. The page states it, and there is no upload step anywhere in the flow — you pick a format and get a
blob:download link.
A four-step walkthrough
Step 1 — drop files in. Click the dashed area or drag images onto it. Multiple files are fine.
Step 2 — pick the output format. Each queued file shows its name, size and a Ready badge. Choose JPG, PNG, WebP or AVIF from the "Convert to" row. Nothing is converted yet, so changing your mind costs nothing.
Step 3 — convert. Press "Convert images". Each row updates in place with input size → output size, a Done badge and a Download link. Failures are shown per file rather than as one vague error.
Step 4 — take the files. Download them one by one, or hit "Download all" for a ZIP. Output names are the input names with a new extension: en-01-overview.png becomes en-01-overview.jpg, IMG_4821.heic becomes IMG_4821.jpg. Your naming survives the round trip.
What happened when I fed it six files
I ran a batch through Chrome on Windows: a 1600 × 1067 photo with EXIF, two PNG screenshots, two HEIC samples and a 400 × 300 PNG with transparency. Output: JPG.
| Input | Before | After (JPG) | Verdict |
|---|---|---|---|
Photo with EXIF (demo-photo-exif.jpg) |
309.1 KB | 403.4 KB | grew 30% |
UI screenshot (en-01-overview.png) |
236.0 KB | 77.6 KB | shrank 67% |
Small diagram (zh-03-technical.png) |
35.7 KB | 53.6 KB | grew 50% |
iPhone-style HEIC (sample.heic) |
286.7 KB | 630.1 KB | grew 120% |
Second HEIC (sample2.heic) |
701.3 KB | 454.4 KB | shrank 35% |
| Transparent PNG, 400 × 300 | 4.7 KB | 6.3 KB | grew, alpha flattened |
Two useful conclusions, both slightly uncomfortable:
Conversion is not compression. A JPEG that was already saved at a modest quality can get bigger when re-encoded, because the browser writes it at its own default quality. WebP behaved the same way on the photo (309.1 KB → 465.5 KB) while shrinking the screenshots (236.0 KB → 57.8 KB, 35.7 KB → 27.0 KB). If your goal is "smaller file", converting first and then compressing is the wrong order — use the Image Compressor or Image Resizer for that job, and convert only when the format is the problem.
Transparency does not survive JPG. The 400 × 300 transparent PNG came out as a JPG with a white background — I checked the corner pixels: 255, 255, 255. Not black, which is the usual ugly surprise. If you need the transparency, output PNG or WebP instead.
Four limitations worth knowing before you rely on it
- AVIF output depends on your browser. Chromium on this machine cannot encode AVIF from a canvas, and the tool says so per file: "This browser cannot encode AVIF. Choose another output format." That is the correct behaviour — it fails loudly instead of writing a broken file — but it means the AVIF button is not a promise on every browser.
- No quality, no dimensions. You choose a format, not a bitrate. If you need 200 KB exactly, or 1200 px wide, convert with this and resize/compress after, or start from the Image Resizer.
-
HEIC decoding can pause. HEIC is not natively decodable in most desktop browsers, so the tool ships a decoder. The first HEIC in a batch can sit at
Processing 3/6…for a while before finishing. My six-file batch did complete; the wait was the decoder warming up, not a hang. - It is memory-bound, not server-bound. Fifty 40-megapixel photos in one batch means fifty decodes in one tab. Batching twenty at a time is kinder to your laptop than dragging in a whole photo library.
Which tool to reach for
The converter is one of a set that all run locally, and they compose badly if you pick the wrong one first:
- Format is wrong → Image Converter (this one)
- File is too big, format is fine → Image Compressor
- Dimensions are wrong → Image Resizer
- Just an iPhone HEIC photo → HEIC to JPG, a one-format shortcut
- Suspicious about what the file carries → Image Info Viewer and Remove EXIF Metadata
A sane order for a photo bound for the web: strip metadata, resize to the width you actually need, then convert or compress. Doing it in that order keeps quality and avoids re-encoding the same pixels three times.
Try it
Open onlitools.com/image-converter, drag in three files you already have, and watch the before/after column. Nothing is uploaded, so the only thing you can lose is thirty seconds.
Screenshots were taken from the live tool in Chrome. The sample photo and HEIC files used in the test are generic samples, not personal images.
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