I am having the same problem and am trying out some of the solutions suggested here. But the way I encountered this problem was interesting and I thought I should share it.
My story goes something like this:
I was testing out OnePress
to integrate WordPress and phpBB3. Everything seemed to work fine except a problem occured in that it would throw 500 internal server errors whenever people tried to change their passwords.
I tracked down the problem to our server and added the following line to the .htaccess file:
AddType x-mapp-php5 .php
As I understand it, this forces my Apache server to use PHP5. By default, it was using PHP4, and the mod I had needed PHP5.
However after that, I started getting the image tag error mention in this thread, i.e. - "It was not possible to determine the dimension of the images." whenever size restrictions were set on the board.
So maybe it also has to do with php versions? Hope this helps some people.