Ok first off. Wrong forum.
Try here.
Or you could
look here on Stepmania.com
To do a background simply put a picture in to the song folder. Let's say your song was called Quickening, you would lable the background Quickening-bg.
Background images are supossed to be 640x480, but you can put an image of any size in the folder, and SM will automatically resize it to fit.
Ont thing: After you put a background, or banner into SM go into SM, and go into edit/sync songs, and find the song, and then go into bg changes. Then scroll down to where it says 'add change to still song,' then exit the editer, and go play the song to make sure the bg working.
NOTE: I don't think the last step is mandatory, but for some reason it won't work on mine unless I do that.
If you have questions on banners check the above links, or PM me.