As you surmise, it may have something to do with the file size. I Think the limit is set by the Web Master to conserve web space. Note the requirement on the R65 site is to keep the size below 384 KB!!!
2 MB is a pretty large photo. Try using a photo editing program, like Adobe Photoshop, to re-size an example below the 384 KB size and try posting. If you don't have a program to handle it, there are probably other members who can suggest a low cost (or free) alternative.
Good luck,
Ed