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These three get used interchangeably and should not be. Resizing changes the pixel dimensions: a 4000x3000 photo becomes 1200x900. Compressing keeps the dimensions and reduces the data used to describe each pixel. Converting changes the container format without necessarily changing either.

Which one you need follows from why the file is too big. If an image is being displayed at 800 pixels wide but the file is 4000 pixels wide, resizing is the fix and it will cut the file size by roughly 95 percent with no visible difference at all. Compressing a 4000-pixel image instead keeps all those wasted pixels and degrades every one of them. Conversely, if the dimensions are already right and the file is still heavy, compression or a change of format is what is left. Resize first, then compress — doing it the other way round throws away quality you then discard anyway.

The choice is driven by the content of the image, not by preference. JPG is for photographs. It uses lossy compression tuned for the smooth gradients in real-world scenes, and it cannot store transparency. PNG is for graphics with hard edges — logos, screenshots, diagrams, anything with text in it — and it is the only one of the three that is lossless and universally supported with transparency. Save a photograph as PNG and you get a file several times larger for no visible benefit; save a screenshot as JPG and the text edges acquire visible fuzz.

WebP does both jobs and produces files roughly 25 to 35 percent smaller than the equivalent JPG or PNG, with support in every current browser. Its limitation is outside the browser: some older desktop software, printers, and government upload portals still reject it. Use WebP on websites, and keep JPG or PNG for anything being emailed, printed, or submitted to a form.

JPG and WebP are lossy, which means every save discards information permanently. Opening a JPG, cropping it, and saving again re-compresses the already-compressed data, and the artefacts from the first pass get treated as real detail and compressed again. Repeat this five or six times and the image visibly degrades, an effect known as generation loss. PNG does not have this problem, because it is lossless.

The practical rule is to keep the original and make every edit from it, rather than editing an edit. If a photo has to be cropped, resized, and converted, do all three in one pass from the source file. And when a workflow needs several rounds of editing, work in PNG throughout and convert to JPG or WebP only as the final step.