First of all the reason why some images why upload is because they are massive.  Most cameras are set to the take the best quatily picture as a default.  This makes the image excessively huge and slows down our server.

 Well, luckily the internet is an amazing thing.  They have websites that will do this for you!  http://www.picresize.com/

Just upload to their site and wait for it to process and save the results.  After that you should be able to upload the new resized image without a problem. 

 It actually goes by file size.  But yes, that is a good guideline to go by.