What compression actually removes
Lossy compression does not shrink a file by storing the same picture more cleverly. It decides which information the eye is least likely to miss and discards it: fine detail inside small blocks, and roughly half the colour resolution, because human vision is far more sensitive to brightness than to hue. What comes back out is a close approximation, not the original.
That is why the same quality setting behaves so differently on different images. A photograph of foliage or skin hides the loss completely. A screenshot, a logo or anything with text sits at the opposite extreme — hard edges are exactly what the algorithm handles worst, and they pick up the faint halos known as ringing.
Where to set the quality slider
Around 80% is the working default and the range 75–85% covers almost every real use, typically cutting a photograph by 60–80% with nothing visible at normal viewing size. Below roughly 60% the block structure starts to show on flat areas like skies and walls.
Above 90% you are mostly buying file size. The curve is steep at the top: going from 85% to 95% can double the file for a difference invisible outside a pixel-level comparison. If an image contains text or line art, that trade is worth making; for an ordinary photograph it is not.
Compress once, from the original
Every lossy save re-runs the process on whatever it is given, so compressing an already-compressed file compounds the damage. The artifacts from the first pass become detail the second pass tries faithfully to preserve, which wastes bits on noise and degrades the image further.
Keep one master file and export from it each time you need a different size or quality. If you only have a compressed copy, compress it as little as possible — and never run a file through repeatedly hoping to squeeze more out, because that is how images acquire the mottled look of something forwarded too many times.
How much size you actually need to save
Work backwards from the constraint. Email attachments generally cap around 20–25 MB; most web forms are stricter. For a website, images below roughly 200 KB keep pages responsive on mobile connections, and the largest image on the page is usually the one deciding your Largest Contentful Paint score.
Resizing often beats compressing. An 8-megapixel photo displayed in a 600-pixel-wide column is carrying more than ten times the pixels it can show; reducing the dimensions first, then compressing, produces a far smaller file than quality reduction alone and looks better doing it.