How the photo gets under 100 KB
The tool first lowers the JPG quality until the file is 100 KB or less. Only if its lowest quality is still too large does it make the picture smaller, and then it says so under the preview. To keep the pixel size, untick “Allow smaller dimensions to meet the limit” under Form requirements and format.
The download is a new file made in your browser from your photo. Your photo is never uploaded, and location and camera details (EXIF) are left out of the download.
KB or KiB
Here 100 KB means 100,000 bytes. Some forms count a kilobyte as 1,024 bytes; for them choose KiB in the unit menu, and the limit becomes 102,400 bytes, 2,400 bytes more. If the form does not say, keep KB: a file under 100,000 bytes passes either way.
If the form also sets a minimum or a pixel size
Open Form requirements and format and enter the smallest size the form accepts, its width and height in pixels, or both. A photo that is already smaller than the minimum needs the opposite job: increase image size in KB saves it again with less compression. Meeting the size does not mean a form will accept the photo, so check its other rules too.