Image formats explained

What each format stores, what it is good for, and what this particular server can do with it. The capability columns are read from the running host rather than from a list, so they describe what will actually happen when you upload a file.

Reading this table. "Read" and "Write" describe the server. Your browser is a separate decoder, which is why a conversion can still work when the server column says no: WebP and AVIF in particular are usually handled by the browser, and the tool reports which side did the work for each file.

Format	Stores	Read	Write	Good for
PNG Portable Network Graphics	Lossless, with alpha	Yes php	Yes php	Screenshots, diagrams, logos and anything with transparency or sharp edges. Stores every pixel exactly, so repeated saves cause no loss. Files are large for photographs.
JPG JPEG	Lossy, no alpha	Yes php	Yes php	Photographs and any image where file size matters more than exact pixels. Cannot store transparency, so transparent areas become a solid colour.
WebP WebP	Lossy or lossless, with alpha	Yes gd	Yes gd	A modern replacement for both PNG and JPG on the web. Usually smaller than JPG at the same visual quality. Older software may not open it.
GIF Graphics Interchange Format	Lossless palette, single transparent index	Yes php	Yes php	Flat graphics with few colours, and animations. Limited to 256 colours per frame, which makes it a poor choice for photographs.
BMP Windows bitmap	Lossless, effectively uncompressed	Yes php	Yes php	Simple, universally readable Windows format. Files are very large because there is almost no compression. Useful as an interchange format, not for storage or the web.
TIFF Tagged Image File Format	Lossless container, optional compression	Yes php	Yes php	Scanning, print and archival workflows. Supports multiple pages and several colour spaces. A container rather than a single codec, so support for a given file varies.
ICO Windows icon	Container, usually with alpha	Yes php	Yes php	Application and website icons. Holds several sizes in one file so the operating system can pick the right one. Maximum size is 256 by 256 pixels.
PDF Portable Document Format	Document, one image per page	No	Yes php	A document rather than an image format. Each selected image becomes one page, which makes it the right choice for sending several pictures as a single attachment, or for scanning paperwork into one file. A PDF page has no transparency, so transparent areas are composited onto the chosen background colour. Choose "fit" page size to keep each page exactly as wide as its picture, or A4 or Letter to place the picture on a standard page. Several images can also be combined into one multi-page PDF from the file list.
AVIF AV1 Image File Format	Lossy or lossless, with alpha	No	No	Very strong compression, noticeably better than JPG at low bitrates. Encoding is slow and support is still uneven in older software.
HEIC High Efficiency Image Container	Lossy or lossless, with alpha	No	No	The default photo format on recent iPhones. Smaller than JPG at similar quality, but reading it requires an HEVC decoder, which many platforms and programs do not ship.
SVG Scalable Vector Graphics	Vector, not raster	No	No	Logos, icons and illustrations made of shapes rather than pixels. Stays sharp at any size. Rasterising it produces a fixed number of pixels, so larger output means more detail.

Why some formats cannot be converted here

Two things have to be available for a conversion: something that can read the input, and something that can write the output. Your browser supplies both for the common formats, which is why almost everything works. For the rest, the server needs an image library with the right codec compiled in.

HEIC needs a HEVC decoder, usually libheif through Imagick. Many Linux distributions do not ship it by default for licensing reasons.
AVIF needs libavif or a very recent browser. Encoding is significantly slower than decoding.
SVG is not raster data, so it needs a vector renderer such as librsvg. Your browser is a complete renderer, which is why this site rasterises SVG on your device by preference.

When neither your browser nor the server can do a conversion, the tool says so and explains what is missing. It will not rename a file and call it converted: a HEIC renamed to .jpg is still a HEIC file, and every program you open it in will continue to see it that way.

Lossy and lossless in one paragraph

PNG, GIF, BMP and TIFF store the image without discarding information. JPG, and WebP and AVIF in their default modes, discard detail the eye is unlikely to notice in order to make the file smaller. Converting a lossy file to a lossless format preserves exactly what is left and cannot restore what was discarded. Converting a lossless file to a lossy format applies the loss once, at the quality you choose.

Choosing a target format

If you need	Use
The smallest file for a photograph	AVIF, then WebP, then JPG
Transparency	PNG, WebP or AVIF
Maximum compatibility	JPG for photographs, PNG for graphics
Exact pixels preserved	PNG or TIFF
An icon file for Windows or a favicon	ICO
An image for print or archival storage	TIFF or PNG