Resize an image for a website

Because every browser in current use decodes it, and it is meaningfully smaller than JPEG at the same visual quality - typically a quarter to a third less for a photograph. That saving is free: the reader sees the same picture and the page loads sooner. The one reason to stay on JPEG is a workflow downstream that cannot read WebP, which is now rare.

Rarely more than about 1200 pixels. A text column is seldom wider than that even on a large monitor, and the browser simply downscales anything bigger while still having downloaded every byte. The exception is a full-bleed hero, which spans the whole viewport and justifies going up to around 1920.

They matter less than the arithmetic suggests, because a phone's layout width is small: a full-width image on a typical phone occupies around 400 CSS pixels, so even at three times density it needs about 1200 real pixels - the same figure as a desktop column. Serving several sizes with a srcset is the thorough answer, and one well-chosen size is a large improvement over an unresized original.

No. They are working figures for a page that should feel fast, and the page labels them that way. Nobody publishes a limit for an image on your own site; what exists is the consequence - every extra 100 KB is time your reader spends waiting, especially on a phone connection.

For a handful, yes, because you get to choose the crop. For a whole gallery, use the bulk resizer instead: it applies one setting across up to a hundred files and gives you a ZIP. This page is the right one when the image matters enough to look at individually.