This would be a good spot for that sort of question.
I am not sure where the setting is, if it even is a setting but yes, it is doable in at least two ways that I know of.
A. Make an image of the same dimensions and name as the default image you are trying to replace, then upload it to the proper location on the server. That way when the page loads it will load your new graphic instead of the default.
That is one way to do it without modifying templates or settings.
B. Modify the templates for your new image that contains dimensions other than the defaults.
I am sure the phpbb site has a lot more information on this topic in their forums.
http://www.phpbb.com/community/