In the visual studio properties window for the UltraToolbarsManager go to Ribbon -> ApplicationMenuButtonImage property.

Ribbon-related issues should probably be directed to the WinToolbars forum; it is actually a separate product from WinExplorerBar, and you'll probably get an answer faster since people who are most familiar with that product monitor that forum. I'm not certain what element you are referring to  here, so if you can, post a screenshot and we will try to help.