Every photo compressor makes the same trade: smaller file, some quality lost. The difference between a compressed photo that looks fine and one that looks visibly wrecked almost never comes down to the tool — it comes down to how much compression was applied and whether anyone checked the result before using it.
Why quality drops unevenly
JPEG compression works by discarding detail that's harder for the human eye to notice — subtle color gradients, fine texture — and it does this more aggressively as the quality setting drops. At high settings (80-95%), the loss is essentially invisible. Below about 50%, you start seeing blocky artifacts, especially around sharp edges and text. There's no single "correct" number — it depends on the photo and how much it's being shrunk by.
The one-slider way to find it
Rather than guessing a quality percentage blind, an image compressor with a live preview lets you drag the quality slider down while watching both the image and the resulting file size update in real time — stop as soon as you can see any softness creeping in, and you've found the actual floor for that specific photo.
When you need an exact file size, not just "smaller"
Exam portals, visa applications, and government forms often demand a photo under a specific limit — under 100KB, under 50KB — rather than just "smaller than before." For that, compressing to a target size automatically searches for the highest quality that still fits under the limit, instead of trial-and-error re-uploading.
Resize first if you can
Quality compression alone has limits — a 4000px-wide photo will always be a bigger file than a 1200px one at the same visual quality. If the photo's final use doesn't need full camera resolution (a web page, a form upload, an email), shrinking the pixel dimensions with an image resizer first, then compressing, gets a smaller file with less visible quality loss than compression alone.