You want to be creating a 1x15 canvas on a program that allows you to do pixel art. Decorate that small strip however you want, upload it as your guild picture and then set it as your background. You can make it move by animating it (which will create a .gif file) and then compressing it into a .jpeg. Here's a quick stationary one I made as an example. Feel free to download it to use as a template (you'll need to zoom in on your browser to download them).
< It's small, but that's how big you want it; or else it won't fit! It's also very easy to customise, so that's a plus.
< Also animated it to show you how it'd look if you decided to add movement.
(FYI, the code to set your guild picture as your background is <body background=profilelogo_GUILDNAMEHERE.jpeg>)
Hope that helps =)