Sparky - one unsolicited idea or at least concept I will throw at you with some background...
I am a bit of photographer and belonged to many photography forums for years. Server space is ALWAYS a big deal. Even Amazon can't afford to give it away for free, and just sold off DPReview, one of the oldest and largest photo community sites on the internet.
As you noticed, the issue is pictures, not text. The way most forums handle the issue is this:
1) Membership and posting is free, BUT, if they want to house pictures on the site and post them from there, they get a VERY basic storage allowance with their free account, if they go over, they can't post more until they delete some of their pics, or go with...
2) an annual paid upgraded account that includes storage upgrade to the basic free account.
3) MOST sites, including yours, allow you to embed photo links. I use this method a lot. This allows to me store my photo's wherever (Flickr, etc) and embed a link to them in my post. This takes up ZERO space on your forum. The only downside here is if I close my Flickr account someday, then my posted pics using links go bye bye. Well, so be it. Not the end of the world for the forum.
So that gives a typical forum user 3 options, use the free account, pay for an upgrade, or host your own files elsewhere and link them here. This is pretty much industry standard practice.
You can tweak the concept 9 ways to Sunday, but you get the general idea.
Hope that helps triggers some idea's for you.
EricV