Why the same photo can be three very different file sizes
Save the exact same image as PNG, JPEG, and WebP and you'll get three noticeably different file sizes, sometimes by a factor of five or more — and none of them is simply "wrong." Each format makes a different trade-off between quality, transparency support, and compression, and the right choice genuinely depends on what the image actually is and where it's going.
Defaulting to whatever format your camera or design tool happens to export by habit means leaving real file-size savings (or needed features like transparency) on the table more often than not.
The three formats, honestly compared
PNG is lossless — zero quality loss, ever — and supports transparency, which makes it the right call for logos, screenshots, and graphics with sharp edges or text where any compression artifact would be visible. The cost is file size: PNG files are typically much larger than the alternatives for the same image, especially for photographic content with lots of subtle color variation.
JPEG uses lossy compression tuned specifically for photographs, producing much smaller files at a quality level that's usually indistinguishable from the original for photo content — but it has zero transparency support, and repeated re-saving compounds quality loss over time.
WebP is the newer format that generally gets the best of both: smaller files than JPEG at comparable quality, plus optional transparency support like PNG. The only real downside is slightly less universal compatibility with very old software, though every current browser and most modern tools support it fine.
A quick way to actually decide, per image
Ask two questions: does this need transparency, and is it a photo or a flat-color graphic? A logo or icon needing a transparent background is PNG territory (or WebP if file size matters more than universal compatibility). A photo going on a website where load time matters is JPEG or WebP territory, since PNG would needlessly bloat the page for content that doesn't benefit from lossless compression anyway.
The one format switch that surprises people the most: converting a large PNG screenshot to WebP for sharing or embedding often cuts the file size dramatically with no visible quality difference, simply because PNG's lossless approach is genuinely overkill for most screenshot content.
Converting from something other than these three
Plenty of images arrive in other formats — GIF exports, BMP files from older software, or camera formats a browser can still open and decode even if they're less common as a delivery format. As long as your browser can display the source image, it can be converted to any of PNG, JPEG, or WebP as the output — the tricky part is rarely reading an unusual input format, it's picking the right output for where the image is actually going.