Compress images without uploading them

The pictures people compress are less innocent than they look. Product photos before a listing, yes, but also screenshots of an internal dashboard heading for a public issue tracker, and a photographed form heading for an upload field with a two megabyte cap. A screenshot of a work system is company data in an image wrapper.

Pick the preset for the destination rather than by size alone. Web Optimized caps the width at 1920 and holds quality at 80, which suits a page image. Social Media produces a fixed 1200 by 630 frame, the shape link previews use, so compose for that crop rather than expecting your whole photo to survive. Thumbnail and Email go much smaller and much harder, and at quality 60 at 800 pixels wide, small text in a screenshot stops being readable.

Format matters as much as quality. WebP and AVIF hold detail at smaller sizes than JPEG; JPEG is the one every system accepts; PNG is the right answer for flat colour, diagrams and anything containing text. An animated GIF and an SVG are refused outright, because a GIF is a sequence of frames and an SVG is markup, and neither is what a still raster compressor operates on.

You can watch the compression run with no network at all. Load the page, disconnect, then compress: it completes and downloads, because the work is happening on your machine. Doing it with the network panel open shows the same thing from the other direction.