Well easy way i think would be.
1. double click on the sig you want and it will come up on its own.
2. now right click on it and save picture as
3. save it somewhere on your comp you will remember!!!
4. go into my XI profile
5. Go to bottom of page and click edit
6. go to the signature portion and hit the little image icon that looks like a tree
7. window comes up and there should be a little button in the upper right hand, click it
8. then another window comes up, hit the browse button on bottom left
9. in your comp pick out the file you saved of the picture
10. hit upload button
11. hit apply changes button at bottom
12. hit the insert image button on bottom
13. click ok on the pop up window
14. click save on bottom of your profile page
15. YOUR DONE
Hope that is detailed enough to get you through it. Let me know if you have trouble.