I have seen several posts with great icon sets for the Perspectives, and I am really enjoying customizing my OmniFocus with icons.

My question for you: is there a trick to what sort of icons you can use? I love the sets people have posted, but I have several lists that deserve specialized icons and I would love to be able to use the range of icons out there beyond this forum, but I can't get it to work.

Is this possible? Is there a trick?
Thanks!