First you'll probably need some kind of photo editing software (Adobe, Paintshop Pro, etc.) and the resize the photo you wish to use down to no bigger then 150 x 150. Make sure the photo also isn't over 19.5KB in size, or it won't accept it.
Once you've sized it down, you then go to your Control Panel (UserCP) here on FlightInfo, and click on "Edit Avatar" on the left-hand pane.
When that page pulls up, simply scroll down to the bottom where it says "Upload photo from my computer" and select the file you just resized. Click "Save Settings" below that, and it'll then show up as your avatar.