A screenshot is not a photograph. It has large areas of identical colour, straight edges and small text rendered at a fixed pixel size. That combination rewards different choices, and treating a screenshot like a photograph is why compressed screenshots so often come back with smeared, unreadable text.
Why JPEG is the wrong tool here
JPEG is designed for continuous tones. Around sharp edges it produces ringing and fringing, and in flat areas it introduces faint mottling. On a photograph nobody notices. On a screenshot, those artefacts land precisely on the letterforms, and the text stops being crisp at a file size that is often no smaller than a properly handled PNG.
What to do instead
- Keep it PNG if the screenshot is already a reasonable size. Lossless storage is exactly right for flat colour and text.
- Convert to WebP when you need a smaller file for a web page. It handles both sharp edges and flat colour far better than JPEG, and it keeps transparency.
- Reduce the dimensions only if the screenshot is larger than it needs to be. Do not scale text down and then expect it to stay readable - it will not.
- Avoid quality below about 80 if you must use JPEG. Lower settings destroy text well before they noticeably shrink the file.
The dimension trap
Retina and other high-density displays produce screenshots at twice the logical pixel size. A capture of a 1200-pixel-wide window can arrive as a 2400-pixel image. If it will be displayed in that 1200-pixel window, halving it is lossless in appearance and quarters the file - the single best saving available for a screenshot.
This is safe precisely because it is not a quality reduction. Text at 1200 pixels displayed at 1200 pixels is as sharp as text at 2400 displayed at 1200, on any ordinary screen.
Transparency
Screenshots of windows often carry transparent rounded corners. JPEG has no alpha channel, so those areas become a solid colour - usually white or black, depending on the tool. If the transparency matters, PNG or WebP are the only correct choices, and the compressor will tell you when a format cannot keep it.
Trying it
Compress a PNG and compare the before and after sizes. For a screenshot that is already reasonably sized, the honest answer is sometimes that the file is fine as it is - Why is my PNG file so large explains when that is the case.