I'm not sure why it won't let you add more than two images. If the third image is larger than 2 MB or has a height/width of more than 2,048 pixels, it won't work because it goes over the size limit. But aside from that, it should be working, so I'm not really sure what to do about that ^^; sorry.
Maybe some more details could help (someone else might know the answer if it's something specific). When you click the "Upload a File" button, what happens? Do you get an error message or anything like that, or does nothing happen at all?