Just take the FBAnext.xpz from "media" folder in the "Fba next" directory in your xbox, open it with xpz tools 2 and extract the folder srcintfSkinfbanext-bg-all.png to your desktop. Replace the wallpaper with the picture you want and and put the folder into the xpz tools again. Transfer your FBAnext.xpz to xbox and see if works.