Why images are almost always bigger than they need to be
A modern phone camera produces images of twelve megapixels or more. Displayed full-screen on a laptop you are looking at roughly two megapixels; in a document or an upload form, often far less. Most of the data in a typical photograph is detail that will never be seen by anyone who looks at it in the context you are sending it for.
That gap is why compression works so well. Reducing quality slightly and re-encoding removes information the eye was not going to notice, and a four-megabyte photograph frequently becomes a few hundred kilobytes that looks identical at the size anyone will actually view it.
It matters because size limits are everywhere and they are strict. Email attachments, job application portals, government forms, forum uploads and content management systems all cap what they will accept, and the caps are often set at values that predate phone cameras entirely.
Quality settings and where the knee is
JPEG quality does not degrade smoothly, which is useful to know. Between 100 and about 80 the file shrinks dramatically while the visible difference stays negligible. Between 80 and 60 the file shrinks more slowly and artefacts start to appear in flat areas — skies, walls, skin. Below about 50 the damage becomes obvious: blocky patches, coloured fringes around edges, and mush where fine texture used to be.
The practical implication is that quality around 75 to 80 is the sweet spot for almost everything. It captures nearly all the size reduction available while staying visually indistinguishable from the original in normal viewing. Pushing to 40 to hit a limit buys relatively little extra saving for a great deal of visible harm.
If you are far above your target, reducing dimensions is more effective than reducing quality. Halving an image's width and height quarters its pixel count, which cuts far more than any quality setting can, and a 1200-pixel-wide photograph looks perfect in a document that displays it at 600.
Hitting a specific kilobyte target
Forms that demand a photograph under 100 KB, or a signature under 20 KB, are common — Indian examination portals are particularly fond of them, and they reject anything over the limit without explanation. Because the rejection rarely says which rule you broke, it is worth hitting the target with a little room to spare rather than landing exactly on it.
Work in the right order. Crop first to remove everything that is not the subject, since you cannot compress away pixels you did not need. Then reduce dimensions to roughly what the form asks for, if it specifies them. Only then reduce quality, and only as far as you must. Doing it in this order reaches any given target at visibly better quality than reaching for the quality slider first.
Check the result at full size before submitting. A photograph that meets the limit but is too degraded to identify a face defeats the purpose of the requirement, and having a submission rejected for legibility after passing the size check is a wasted round trip.
Formats, transparency and the mistakes that cost you quality
JPEG is right for photographs and wrong for screenshots, logos and line art, where its compression produces visible fringing around hard edges. PNG is the reverse: excellent on flat colour and sharp edges, unnecessarily large on photographs. WebP does both well and is now supported everywhere that matters, though some older upload forms still refuse it.
One trap is worth naming explicitly. Saving a JPEG repeatedly compounds the damage, because each save re-encodes what the previous save already degraded. Editing a photograph five times through five JPEG saves produces visibly worse results than making all five edits and saving once. Where you can, keep an untouched original and always compress from that rather than from your last compressed copy.