Ok, as a test to see if I could change it back to my old avatar (the answer was No). here's some screenshots as I go through the process...
My profile page when I started, showing the current avatar you loaded for me:
The Page I get when I select "Modify Avatar"
The "Modify Avatar" Page after I've browsed to the photo and hit "Update" The "Current Avatar" photo was replaced with the one I had just browsed to.
Then I hit the "Back" button at the bottom of the page, just like you did and here's the page I get, with original avatar photo that I was trying to change.
capture-20130825-231201.png
You have insufficient rights to see the content. capture-20130825-231245.png
You have insufficient rights to see the content. capture-20130825-231410.png
You have insufficient rights to see the content. capture-20130825-231441.png
You have insufficient rights to see the content.