Read the form wording first
Exactly 100KB and 100KB or less are different requirements. Use exact mode only for a fixed target. Use maximum mode for a ceiling, because a smaller valid result does not need extra filler bytes.
Select format and allow safe reduction
JPG is a practical choice for photos. PNG is useful for transparency and sharp graphics, but may be larger. WebP can be efficient when the receiving portal accepts it. Keep dimension reduction enabled unless the form also requires fixed pixel dimensions.
Verify before submitting
Download the processed image only after the result shows a successful size badge. The tool reopens the output and checks Blob.size, but the destination portal still decides which formats and dimensions it accepts.