You might have noticed that we have a Profile Page which has the option for both a profile picture and a cover picture. We currently don't have it programed to be able to upload a picture from your computer. BUT, what you can do is link to an already existing image, that's posted anywhere on the web--including here on permies.
First, you find the image that you want (if you don't have one online, you can always attach it to one of your posts), right-click on it, and select "Copy Image Address"
Then, go to your profile settings and paste that URL into the form for either "Profile Picture" or "Cover Picture." Scroll to the bottom of your settings and click "Submit." You're new profile &/or cover picture should now be visible!