I found this out, using my MP3 player, I find an image file, copy it to my MP3 player the go to Pictures on Xbox dashboard. Press X select Portable device and it has your images on there and you can use them as backgrounds for your Xbox.
you can only listen to music, watch movies/videos from a USB on ur 360..or if you want you can get the new 08 update by puttin that file onto ur USB then into ur 360..
it would be much harder to do that then just plug it in xbox and put files on it you would need to get files on your xbox like hotswapping them on there or modchip should work but hotswap is free but much harder modchip is one time install just if you get a modchip you need make sure its off when you get on xbox live or your ****ed banned! so try to get files on comp then put on your usb from there but you still would have to do it again on other xbox you wanna put stuff on