I had a picture as a buddy icon and wanted to change it to reflect the change in the season but whenever I went to change it, it would go back to the old picture. I was able to turn it off but whenever I chose a new picture it would again go back to the old one. Here is what I did to fix that.
Navigate to C:\program files\trillian\default and look for a file called buddyicon. This won't be the actual buddy icon but rather a file called buddyicon. Right click on the file and click on properties. Un-check the read only box and click ok. You will now be able to change your buddy icon.