Why 100 KB is a popular limit
Many websites and forms accept photos up to 100 KB because it keeps their servers fast while still allowing a sharp picture. Modern phone photos are 2–6 MB, so they need to be reduced before uploading.
What happens when you resize
The photo is saved as a JPG with the best quality that fits under 100 KB. iPhone HEIC photos are accepted and saved as JPG automatically. If the form asks for specific pixel dimensions, enter the width and height; otherwise the photo keeps its shape and is only made smaller when necessary.
What quality to expect
With 100 KB, most photos stay clear even at a good pixel size, such as about 1000 pixels wide. The photo is never cropped — only its file size and, if needed, its pixel size change. For ID photos with fixed millimetre sizes, use Passport Size Photo Maker first.
Common problems and how to fix them
The form rejects the photo's shape: crop it to the required ratio with Crop Image before resizing. The photo is sideways: rotate it with Rotate Image first. The limit is smaller than 100 KB: use Resize Image to 50KB or 20KB.
Good habits
Use good light and a plain background, keep the original photo, and check the KB size shown before uploading.