Ok, it's "easy" in terms of how to do it, but not as easy as it is for the default 4. Install filebox, and you need to name the image 'background-5/6/7/8/9.png' it HAS to be .png. Use filebox to put it in the .backgrounds folder under opt/usr (i think. i'm doing this from memory so i may be a bit out, but i'm pretty sure it's opt/usr/.backgrounds). the '.' before 'backgrounds' means it's a hidden folder, so you need filebox to find it and in settings click 'show hidden folders'. That's the folder you need to place backgrounds to change the added homescreens, and they need to be titled 'background-5.png' for example to change the 5th homescreen