A form wants a photo under 100 KB. Your phone produced a 4 MB file. Lowering quality until it fits leaves a smeared mess — but there is a second lever most people never touch, and it does far more work.
That lever is resolution. A modern phone photo is around 4000 pixels wide. The form that rejects it will display the picture at perhaps 600 pixels. You are sending forty times more detail than anyone will ever see, and paying for it in file size.
Start by capping the width at what the picture will actually be shown at. For a profile photo or a form attachment, 1000 to 1200 pixels is generous. That single change typically cuts the file to a fraction of its original size while the image still looks sharp on any screen.
Only then lower the quality. JPEG quality around 70 percent is usually indistinguishable from the original on screen; below 50 percent, edges and skin tones start to break up visibly.
JPEG is right for photographs. PNG is right for screenshots, logos and anything with sharp edges or transparency — a screenshot saved as JPEG looks dirty around text, and a photo saved as PNG is often ten times larger than it needs to be.
WebP is smaller than both at the same visual quality and every current browser reads it. Only use it when you know the destination accepts it: some official upload forms still reject anything that is not JPEG or PNG.
For a normal photograph, a width of about 1000 pixels at 65 to 75 percent quality lands close to 100 KB. Busy pictures — foliage, crowds, fine texture — compress worse than smooth ones, so if you land above the limit, take another 200 pixels off the width before you drop quality further.
Check the result at full size before sending it. A picture that looks fine as a thumbnail can be visibly blocky when someone opens it.
Compression is arithmetic your own device can do, so there is no technical reason to send a personal photo to a server first. Abzar's image compressor runs in the browser: you set the quality and maximum width, watch the resulting size, and download — with no upload wait and no copy of your picture on anyone's server.