phpbb isn't the best when it comes to posting images in thread, it can make the text go funny and disappear off the side. Presumable this is one of the reasons why you've limited the dimensions of user posted images.
Something I installed on another forum was an auto-resizer. It still allows you to post larger images but will auto-resize them in the thread. Links below if you are interested to see and try.
Instructions & download
http://www.phpbb.com/community/viewtopi ... &t=1696215
working example
http://phpbb.davidiq.com/viewtopic.php?f=9&t=353
Something I installed on another forum was an auto-resizer. It still allows you to post larger images but will auto-resize them in the thread. Links below if you are interested to see and try.
Instructions & download
http://www.phpbb.com/community/viewtopi ... &t=1696215
working example
http://phpbb.davidiq.com/viewtopic.php?f=9&t=353