Why photos are large
A modern phone photo has 12 million or more pixels and often weighs 2 to 8MB. A form that allows 100KB needs roughly a 20 to 80 times smaller file.
Step 1: choose the target
Read the limit on the form. Common limits are 20KB, 50KB, 100KB and 200KB. Aim just under the limit, not far below it.
Step 2: compress to that size
Open the matching tool, such as 50KB, 100KB or any custom size. It keeps the best quality that fits and runs in your browser.
Step 3: resize if needed
If the file still looks soft, reduce the pixel dimensions with the resize tool first. Fewer pixels need less compression.
Mistakes to avoid
Screenshotting the photo, compressing the same file again and again, and uploading a PNG when JPG is allowed. Keep your original in case you must start over.