All Tools VerseThe fastest way to tell PNG, JPG, WebP, and GIF apart is to read the first few bytes of the file, called its signature or magic number, not to trust the file extension. A PNG always starts with the same byte sequence, a JPG with another, and so on, no matter what the name says. This guide explains the signatures, why extensions lie, and free tools to check an image’s real type.
In this guide
Why the extension is not enough
A file extension is just part of the name, and anyone can rename a photo.jpg to photo.png without changing a single byte inside. Programs that trust the extension then fail to open it, while programs that read the real bytes succeed. This is why uploads, converters, and security checks look at the signature instead, since that is the part the file cannot fake just by being renamed.
The four signatures
Each format begins with a fixed marker. PNG starts with the bytes 89 50 4E 47, where 50 4E 47 spells PNG. JPG starts with FF D8 FF. GIF starts with the text GIF, either GIF87a or GIF89a. WebP starts with the text RIFF, then four size bytes, then the text WEBP. Reading those first bytes identifies the format with certainty, which connects to our binary and bytes guide.
Check an image’s real type
Rather than open a hex editor, you can use a checker for each format. The is it a PNG tool, the is it a JPG tool, the is it a WebP tool, and the is it a GIF tool each read the signature and tell you yes or no, all in your browser with no upload.
Which format to keep
Once you know what a file really is, you can decide whether to convert it. WebP is usually the smallest for the web, PNG is best for sharp graphics and transparency, JPG suits photos where small size matters more than perfection, and GIF is mainly for short animations. Our format comparison guide covers the trade-offs in depth.
Free tools used in this guide
Frequently asked questions
How do I tell what format an image really is?
Read its first few bytes, the signature, rather than trusting the extension. A format checker does this and tells you the real type.
What bytes does a PNG start with?
89 50 4E 47, where the 50 4E 47 part spells PNG. JPG starts with FF D8 FF.
Why can a file have the wrong extension?
Because the extension is just part of the name and can be renamed without changing the file’s actual bytes.
How does a WebP file start?
With the text RIFF, then four size bytes, then the text WEBP, so reading the start confirms it is a WebP.
Do the checkers upload my image?
No. They read the file’s first bytes in your browser, so nothing is sent anywhere.