No limits of any sort have been published. If they have, they sure haven't been brought to the attention of the Forum owners.
You can always go to www.proboards.com, the main page gives you a clear snapshot of what's what, with links for details. There are currently over 120 ProBoards servers, handing out pages for more than 2 million boards!!!!!! If you do go there, you'll note the commercial aspect of things. They sell space for storing images, that's how they make their money. We don't do that, we more-or-less require that members use off-site storage, and link to images.
Like RandomHero before me, I am extremely loathe to start dealing with money, for any reason. We seem to be fulfulling our mission and our goals just fine as it is, don't we?