Within the first gray, striped banner, you will see a list of links. Click on User CP
In the next page, on the left, there is a column, under a purple banner that reads "Your Control Panel". Under that, there are a few gray banners. Under the gray banner that reads "Settings and Options", the first link listed is "Edit Avatar". Click on that link.
In the center field, it gives you a varaition of options to choose from, including preset avatars to choose from, upload one from another website or the option to upload one from your computer with the button toward the bottom of the field.
The maximum size of the image should be 150x150 pixels or within.