Signatures are currently disabled at the moment.
However, it is possible to have an avatar.
Firstly, you need to click the Edit Profile button next to the 'log-out' button to access your Profile. This is fairly near the top of the page. Then click on the tab which states 'Avatar'. You'll need to Enable Avatars. Then click browse to find the picture within your computer - Presuming you have a picture saved on your system. Then you'll need to click update, and lastly press the save button towards the bottom of the page.
It should work
.