One thing I'd definitely, definitely do is clamp down on people quoting the post above them. The number of times a huge image gets posted and the next 3 or 4 posts quote it in its entirety with just a smiley as their own message take the p*ss really. All of those really start to add up when you're talking about bandwidth.
I don't know about on Ikonboard, but my missus runs a site on phpBB which is very similar - there's a modification you can add to the forum software which stops users being able to quote the message directly above them in its entirety.
Also, maybe look into stopping people using gifs on the site. Some of the animations people use as their avatar or in their signatures are f*cking mahoosive. At the very least I'd seriously consider putting a strict kb limit on people's signatures. It's not hard for people to get some of those jpeg file sizes down.
A more radical solution would be to do away with pics in signatures altogether. I never really see the point to having both an avatar and a signature anyway. Some of the pics people have are great, but it's not what we're here for, and I'm sure people would rather do away with them than contribute to the site going down the pan. If people want a Shankly quote, to link to the HJC site or to give a thought to Drummerphil, there's nothing wrong with good old text.
By all means come up with ways of fundraising, but a bit of tweaking of how people can use the board will save you a ton of bandwidth. The missus tightened up her rules in 3 or 4 fairly subtle ways and her monthly usage almost halved.
I'll have a think on the cashflow front as well fella - it's a great community and I'm sure I speak for all of the regulars by saying that we'd hate to have to take our nonsense elsewhere in the long run.