Besides the file type that was already mentioned, could it be possible that you are accidentally cancelling the upload?
When I first added a picture to my tree, I selected the file from my computer, clicked on "Upload" and then immediately clicked on the next person on the tree to upload their picture as well - which cancelled the initial upload. You have to wait for a few seconds until the picture is uploaded before you can click on another person.
Hope you find the solution for your problem!
