The workshop · Convert an image
How it works : Convert an image
Drop an image in. The tool offers you ONLY the formats your browser can actually write, and it reads the result back to check it really got the one you asked for.
A form that only takes JPEG, a program that has never heard of WebP, a picture that needs to be transparent: changing an image's format is an everyday need. It is also the favourite playground of the converter farms, those sites covered in adverts and fake download buttons, all of which begin by asking for your file.
This tool does the same job while receiving nothing. And it does one thing the others do not: it only offers the formats YOUR browser can really write. That is not a courtesy, it is a technical necessity. The canvas specification allows a browser that cannot write the requested format to return a PNG without warning: a site that lists “WebP” without checking therefore hands a disguised PNG to every Safari visitor.
The type of the file you get is read back afterwards, and displayed. If your browser produced something other than what was asked for, you will know, and the file name will match what it really is.
How it works
- Drop your image on the drop zone, or choose it, or paste it from the clipboard.
- Pick the output format. The list only holds what your browser can write, here and now: it may therefore be shorter on an iPhone than on a computer.
- Click “Convert and download”. The type of the file you get is read back and displayed, and the file downloads.
Formats accepted
These six formats OPEN in every up-to-date browser. On the OUTPUT side the list is shorter, and it depends on the browser: that is why it is measured at run time rather than announced. The compatibility matrix gives the detail, browser by browser.
JPEG photo (.jpg, .jpeg)
Opens and writes everywhere. No transparency: converting a transparent image to JPEG therefore lays a background colour down, and the tool says so before doing it.
PNG image (.png)
Opens and writes everywhere, and keeps transparency. It is the only format the canvas specification makes MANDATORY: it is therefore also the universal fallback.
WebP image (.webp)
Opens everywhere since Safari 14. On the writing side Chrome, Edge and Firefox can do it; no version of Safari can, on Mac or on iPhone. The output list therefore adapts to the browser.
AVIF image (.avif)
Opens everywhere since Safari 16.1. On the WRITING side, however, we found no browser that really exposes it, and no first-hand source for the much-repeated claim to the contrary. AVIF will therefore appear in the output list only if your browser genuinely can: that is measured on your machine, not assumed here.
GIF image (.gif)
Opens everywhere, for reading only. An ANIMATED GIF is refused: a canvas holds one still frame, and converting would keep only the first one.
BMP image (.bmp)
Opens everywhere, for reading only. It is an uncompressed format, so enormous: converting it to PNG often divides it by ten while losing nothing.
Its limits
50 MB per file
Beyond that, a phone short of memory crashes the tab instead of returning a result. The limit is a piece of honesty, not a commercial leash.
About 16.7 million pixels per image
On iPhone and iPad, canvas area was capped at 4096 by 4096 pixels for years, and allocation fails beyond it. The tool checks the area BEFORE allocating and refuses cleanly.
The output formats depend on your browser
Writing an image is a browser capability, not a site one. Safari cannot write WebP, and for AVIF we found no browser that really writes it. Rather than announce a flattering list and quietly return a PNG, the tool measures what this browser can do and shows only that.
Animated images are refused
A canvas holds one still frame: an animated GIF, PNG or WebP would come out as its first frame. The tool detects that in the bytes of the file and refuses, rather than handing back a maimed file without warning.
HEIC photos from iPhones are not read
Only Safari can open a HEIC; elsewhere it would take a decoder of more than ten megabytes. An iPhone can be set to “Most Compatible” to shoot JPEG, or share the photo as JPEG.
Questions about this tool
Is my image sent anywhere?
No, and you do not have to take our word for it. Open your browser's inspector with F12, go to the “Network” tab, clear the list, then drop your image in. Not one line appears. You can also cut your connection: the tool keeps working.
Why is WebP missing from my output list?
Because you are probably on Safari, on a Mac or on an iPhone. No version of Safari can write a WebP from a canvas. That is not a limit of this site: it is a limit of the browser, and it is checked in yours the moment you open the page. On Chrome, Edge or Firefox, WebP is there.
How do you know what my browser can write?
By asking it. The tool creates a canvas one pixel by one pixel and asks the browser to save it in each format, one after another. The browser answers by stating the type of what it wrote: if it announces something other than what was asked for, the format is not supported. That is a measurement, not a list copied from a table that ages.
What happens if I convert a transparent PNG to JPEG?
The transparent areas are filled with white, because the JPEG format cannot represent transparency. The tool detects transparency in your image and warns you BEFORE the conversion, so that you can choose PNG or WebP if it matters to you.
Does converting damage my image?
It depends on the destination format. To PNG, no: PNG is lossless, every pixel is recorded exactly. To JPEG, WebP or AVIF, yes, a little: those are lossy formats, and the quality setting decides how much. In every case it is a real re-encode, never a copy: converting a JPEG to JPEG recompresses it.
Why do you refuse animated GIFs?
Because a browser canvas holds one single still frame. Converting an animated GIF would keep only its first frame, and you would get a mute file back believing you had converted your animation. The tool looks into the bytes of the file to see how many frames it holds, and stops if it finds several. That is a refusal, not a failure.
What about the HEIC format from my iPhone?
It is not read, and that is written into the tool's limits. Only Safari can open a HEIC; elsewhere it would take a decoder of more than ten megabytes, which we refuse for a site that has to work on a weak connection. In the camera settings, the “Most Compatible” option produces JPEG files directly.
Drop an image in. The tool offers you ONLY the formats your browser can actually write, and it reads the result back to check it really got the one you asked for.
VerifyThe proof, step by stepThe network inspector check, explained one gesture at a time, with screenshots.
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