What picture are you referring to? The icon, the MOVIES image, or the sub-menu background?
I know the MOVIES/VIDEOS/WEATHER etc. images are located in the media/textures.xpr file, which can be opened with XPR Express:
http://teamblackbolt.co.uk/projdetails.php?id=5
The file is named <language>-movies-focused.png or something like that, <language> being your set language in XBMC. There's an -unfocused version too. If I recall correctly Home.xml is the file that controls the home screen in Rapier, might be an Include_Home.xml file too, can't remember. But search for the file name from the .xpr inside the .xml files and you should find it. Be sure to leave out the language part (just search for movies-focused.png or whatever you want to find), if that's the file you want to change. Then you either change the image you want to change, and repack the .xpr, or you change the .xml file you found it in, to reflect your own file. Say you put it in the media/ folder and named it foobar.png, you would reference it in the file by putting special://skin/media/foobar.png as the <texture>.
If it's the icon you want to change, it should be in there too but I'm not sure what it's called. But if you find the MOVIES image, you should find the icon file name somewhere in that file too, probably a line or two above or below.
The sub-menu file is probably in that file as well.
I hope this makes any sense, and that I understood the question. I have a cold, so I'm just gonna blame any mistakes on that.
Happy tweaking!