There's two parts to the process, one is straightforward, the other could potentially be challenging, depending on your ability to do some basic image editing.
Click on the User Control Panel
link in the top menu bar, click on the "Profile" tab, then the "Edit avatar" link on the left.
The easy part is telling the forum what picture to use. You can do this one of three ways:
1. Upload picture. If the avatar you want to use is on you local computer, click the Browse button, find and select the file and click the Submit button.
2. Upload from URL. If the image is already out there on the web, you can type in the URL for the image, and when you click submit, the forum will pull the image from the remote site and set it as your avatar. If the image on the other site changes, it won't change here, because the forum will use the copy it made .
3. Link off site. Haven't tried this one myself, but it looks like it will just use the URL for an image that is already out there on the web, I guess if the remote image changes, the changes will show up here as well. That could be useful if you wanted to use the same avatar on multiple forums, and if you linked the one here to one somewhere else, changing it there would result in it changing it here,
The potentially hard part is meeting the avatar file size requirements. There are two limits, image dimensions and image file size. A digital picture is made up of picture elements, or pixels, individual dots. The forum sets a dimension limit "Avatars must be at least 20 pixels wide and 20 pixels high, but no larger than 90 pixels wide and 90 pixels high" That's a fairly small picture. There file size limit is 6KB, or 6144 bytes, which is also small, but reasonable for a picture that is 90x90 pixels. Whatever picture you use as an avatar needs to meet both these limits.
Digital cameras these days take huge pictures compared to these avatar limits, but I wouldn't visit a forum where people had megapixel avatars. To make an avatar, I usually crop and resize a digital picture. Everyone has different skills, and within the full range of digital image editing tasks, cropping and resizing an image is relatively basic, but it can be challenging if you are not used to it.
If you're not familiar with how to crop and resize a picture down to meet the forum's avatar size limits, you can post the picture or a link to the picture, describe how much of it you want to show in the avatar, and members here can do it for you.