Neither Hoop.la nor UBB Forum has any "webspace" for storing files for your site in general. Sure, your community members can upload files as content, but aside from the simple Header Graphic upload, there is no way to store images/files for things like custom widgets or a custom footer for your site.
One simple way to solve that problem is to use a free Dropbox account. Any files you place in your Public directory on Dropbox can be used as "Public Links" for things like web images.
We've created a tutorial that explains how this works. You'll see just how easy it is to use these images/files into links you can use on your site.
Update As Of October 29, 2014:
We just rolled out a new feature for Hoop.la, UBB Forum, and QuestionShark that adds Web Space for storing web resources and files. This means that you no longer need to use a service like Dropbox for that. For details on Web Space, read this post.