Which image tool should you use?
If a file is simply too big, use the image compressor and move the quality slider until the size looks right. If a website or form sets a hard limit, use the exact-size tools instead: compress image to 100KB, 50KB or 20KB find the best quality that still fits.
If a file will not open, it is usually the format. HEIC to JPG fixes iPhone photos that Windows or a website will not accept. PNG to JPG makes screenshots and graphics much smaller, JPG to PNG gives you a lossless copy for editing, and WebP to JPG turns images saved from the web into a format every app understands.
Getting the smallest file without a visible loss
For photos, a quality of 75 to 80 percent is usually impossible to tell apart from the original while cutting the size by half or more. Resizing helps even more: a phone photo is often 4000 pixels wide, while most websites and forms show it at 1200 pixels or less.
Converting to WebP gives the smallest files for websites. Use JPG when the image is going into an email, a document or a form, because every program accepts it.
Why these tools run in your browser
Photos can hold more than the picture: location, the date and the phone model are often stored inside the file. Here the work is done by code running on your own device, so your images never travel to a server, and location and camera details are removed from the output automatically.
The trade-off is that your device does the work. Large batches may take a little longer on an older phone.