Click on "Edit Emoticons" in your admin center (top centre), and at the bottom you'll see "Add New Emoticon:". In the box that says "Name", enter what you'd like to type in to use the emoticon. For example, typing "mysmiley" would let you use :mysmiley: for that one. Then in the box that says "http://", enter the URL to the emoticon. If you don't have them uploaded to web space, have a look in the "Web Hosts" thread in Off Topic here.
The easiest way to add a wallpaper is to click on "Easy Customization" in your admin centre. There's a box for the URL to a background there. But, if you're only
using it for the background, and not choosing colours, fonts etc with it, make sure to check the "don't clobber current CSS" box.
I'd recommend that you choose all your colours there, and do clobber the CSS