Short answer
Lower the image quality, and if that is not enough, reduce its pixel size, until the file fits under the limit. The free reduce image size in KB tool does this in your browser: add the image, tap the limit and download. Nothing is uploaded.
Free tools for this
Two things set the file size
File size depends on the number of pixels and on the quality. Lowering the quality shrinks the file with little visible change at first. Lowering the pixels shrinks it more, but the picture gets smaller. The tool tries quality first and shrinks the picture only if it must.
What each limit looks like
At 200 KB or 100 KB an ordinary photo looks fine. At 50 KB it is smaller and slightly softer. At 20 KB it is clearly reduced, so keep the picture simple, like a face or a signature on a white background.
Try it now: Reduce Image Size in KB → · Image Resizer →
If it will not fit
Resize the picture to the pixel size the form asks for first, then reduce the KB. A photo straight from a phone has millions of pixels, far more than a form needs. A white or plain background also compresses much better than a busy one.
Format matters
JPG is the best format for photos at small sizes and every form accepts it. PNG files are large for photos and ignore the quality setting, so convert a PNG to JPG first.
Check before you upload
Open the result and look at it at full size. Check that the face or signature is readable and that the form accepts the file type. Keep the original in case you need another size.