Just curious, is there a size/dimension limit to the header image?
As a matter of fact, there is.
A forum like CathInfo can't have a huge header (like Non Possumus, for example) because this isn't a blog.
Just look at how much is on the main page, or any thread you click on. Would you want to have to scroll that much more every time you click on "next page" of a topic, a new subforum, a new topic, and so forth?
A blog has far fewer "posts" per day and they all appear one after the other, so you just keep scrolling down, down, down. The size of the header then becomes irrelevant.
A forum is a whole different animal than a blog, despite some people getting them confused. (I still don't understand this confusion -- a blog is a one-person affair, whereas a forum is a place for hundreds or thousands of people to discuss topics, where everyone has an equal voice and an equal prominence.)
It would be like mixing up a screwdriver and a hammer (handyman), diesel and gasoline (mechanic) or a stimulant and a depressant (medicine). Or debits and credits (accounting). Or being on foot vs. on horseback (ranching). You get the idea: Completely distinct.