You'd have to upload it and wait untill Paul approves it or deletes it.
Please don't use images that you don't have permission to use like other people's fanart or movie photos and stills. Has to be something that you clearly have permission from the original artist to use.
If you go to your Profile page, near the bottom where it says "Upload Avatar from your machine.", click the "Choose" button. Then use the resulting window to browse on your computer to where your image is.
It should be possible on any PC running Windows or Linux with a desktop environment (probably Apple Macintosh's as well but I've no knowledge of them and have never used one). If you can say where and how you're running into a problem maybe we can get there step by step.
That needs to be approved by Paul before it'll show up. Maybe you should send him a pm and ask nicely for it to be approved because hes rather busy so it may get missed if you don't remind him to look for it.
That's a nice one, draconeedsahug - can you just confirm whether you created it yourself...or if not, if you could point me to wherever you got it from so that I can check that permission from the original creator has been given.
Sorry draconeedsahug - that site is using many images which would be in breach of copyright so there's no way to know whether they've stolen the image used for your avatar too.