Three presets, or your own numbers
Smallest, Balanced or Best quality to start. Then set the quality, the PNG effort or a cap on the longest side yourself, and pick the format you want out: JPEG, PNG, WebP or AVIF.
Make it smaller
without sending it.
Re-encode a picture on your own machine, compare it against the original at full size, and keep it only if it still looks right.
How this works
Judge it at full size
Compression artefacts are invisible in a thumbnail, which is exactly where most tools show you the result. Here both versions are full size and you can pan around them, so you can look at the places that actually go wrong.
The metadata goes too, and that is checked
Compressing decodes the picture to raw pixels and encodes those, so there is nowhere for GPS, camera serials or hidden thumbnails to survive. blakstrip then re-reads its own output and tells you what it found, rather than asking you to take that on trust. AVIF is the one format it cannot read back, and it says so instead of claiming a check it did not run.
It works with the network off
The encoders are WebAssembly, shipped inside the page rather than fetched when you use them. Turn off your wifi and compress a photo. AVIF is the one exception: it is large enough to be loaded on demand, so it needs one online use first.
Want the metadata gone without touching the picture? Remove image metadata rewrites the container and leaves every pixel exactly as it was.