When highlighting a game, press 2 (IIRC) and you can see the game's description. When you click at the game's box at the left of the description, you can now enjoy the full boxart. You can rotate it with the dpad, zoom in/out with + and - etc.
Also, to see the disc cover, set the relevant option from USBLGX's settings. 3d covers are viewed from the menu when you use the list arrangement (ie not the banner or the grid ones).