What you need:
A GIF or JPG image you'd like to use for your avatar, less than 15K in size on your disk
What to do once you've got that:
On the 'Change Forum Preferences' screen, simply click 'Browse...' and find the image on your system. Select it. Then click 'save forum preferences'. Your image will be uploaded and processed. Note that your avatar will not be immediately visible on the site! It must be "approved" by one of the moderators first. This usually takes less than 24 hours.
Can I use animated GIFs as my avatar?
No, they don't work. Only the first 'frame' of the animation will be saved as your avatar. It's best to stick to static images.
What if my image is bigger than 15K? How to I make it smaller?
You'll need to use an image editing tool such as Photoshop or Paint Shop Pro. First, resize the image down to around 64 x 64 pixels. Any text will be totally unreadable in that size, so keep that in mind. If possible, reduce the number of colours as much as you can, and if your software has a web compression wizard (for example, Paint Shop Pro has an Optimizer wizard that helps you save out files really small), use it. You should then have a nice small avatar image ready to upload!