I'm also guessing it's the size.
If you use photobucket, you can download your photo from there to your computer and it will download at a smaller size.
Some email software give you the option to choose a smaller size when sending a photo, so that is a quick and easy way to resize.
If you take your photo with your phone (or similar), you could resize the photo directly on the phone (mine is in editing tools, then adjustments, where it gives the option to resize).
Finally, pretty much any photo editing software will allow you to resize your photo.
I'm not sure what the size limit is, but as long as my photo has been under 500 kb, I haven't had issues loading them to this site from my computer.