Pick the right format first
JPG is usually the right choice for photographs because it can reduce file size aggressively with little visible change. PNG is better for screenshots, sharp graphics, and transparency, but it may resist tiny KB limits.
WebP can be efficient for many web images, but the receiving form has to accept it. When a portal asks specifically for JPG or PNG, format compatibility matters more than theoretical compression.
Understand what changes
Lossy compression changes image data to save bytes. If quality alone is not enough, the browser may also reduce dimensions. That can be acceptable for thumbnails and profile photos, but not for documents where small text must remain readable.
A compressor should show the measured output size and dimensions so the result can be checked before download.
Avoid unnecessary uploads
For profile photos, forms, application images, and private screenshots, local browser compression avoids sending the original file to a third-party upload server.
The trade-off is device work: larger images may take longer on older phones, because the compression happens on the device instead of a remote machine.