For most of us it is because our file size is too large. There is a good FAQ but the screen shots and options shown don't exist on my system, maybe because of the various levels of security and that I have never installed Java on it. Our camera is older and not so easy to use as modern stuff either, so I take pictures on the camera, transfer them to my quilt photo folder, and then I reduce the file size at this free site that doesn't set off my computer protection warnings:
https://www.reduceimages.com/
You download from that site, to upload here you can simply go to your Downloads folder or save it from there to some other location. Fast and easy and doesn't give you many options but the only option I care about in this case is posting them here.