Depends on what you mean by "menu". If you mean the short menu you see in game when you press escape, then sure, but if you mean the menu before you start the game, then no, as that is not helping the players who are playing that very faction.
And a still image can still have mouse-over tooltips very easily by using an image map. Many webpages can do this, for example (it's a standard part of HTML). The best example I can think of is facebook, where you can tag the people in an image so when you hover over them, you can see their name and click to see their profile page. In this case, it could just raise the standard description by taking it from the faction language file (GAE mods should already have descriptions for units inside this file anyway). Naturally, the tooltips would be optional.