There are two ways to do this.  One, you could do it via your html provider, if it supports this type of functionality. The other way is to set permissions on the gallery exposed to the module. For each gallery in a forum there is a hidden tab created and you can set permissions on that tab.

Avatars and images used in content similar to emoticons are handled seperately. Although both are handled seperately they both currently use the gallery.  My suggstion of the html provider was only for within post content. The FTB html provider does have the ability to show an image button on it, but I have never done it myself. Right now I am trying to figure out what is best to handle both the avatar and image scenario's because I don't think this module should be specifically tied to the gallery module unless the site owner wants it to be. To be honest my time is a bit limited right now to get further into the situation but the problem could be fixed via gallery for now. We are working on gallery as fast as we can right now as a team. We are held up by some personal circumstances, but are communicating.