I'm afraid that's not possible. CSS stylesheets for Flash have limited control of text formatting only. You can not use it to insert images in any way.
Perhaps you can explain exactly what you want to do, and I could see if it might be something we can improve in a future release.
Just found a workaround solution to this - Go to admin -> config -> edit your language. Find the "mainmenu" item, and add some HTML to insert an image: