Hello

First, Welcome VoE!

To get a picture, you need to have a 100x100 pixel picture hosted somewhere (i.e. you need to be able to give UM.com a url like http://my.server.org/something/picture.jpg)

Then click profile in the upper left hand corner.
Click on "edit profile" in the main body of the page.
At the bottom of the edit form, put the aforementioned URL in the "user avatar" field. (don't forget the http://).
and it should work from there.