Problem w/png's and phpbb


I have been doing some testing to verify the uploads on my forum. I have noticed that a 2048x2048 png image uploads fine, however a 4096x4096 png image generates an interal server error. For awhile I assumed this was a server problem. But I then tried the same resolution using a jpeg and it uploaded fine. The file size is even bigger.
I now beleive the problem lays with phpbb or the attachment mod. Has anyone else run across this problem.

Edit: I forgot to mention that the png file is listed as a shadow attachment, meaning it did upload but did not attach itself. Also I am currently using version 2.0.13