The icon was made by chapstick_creo - and can be found in the post here (http://community.livejournal.com/chapstick_creo/6193.html#cutid1). It looks like her usage policy is comment to say you're gonna use it and credit her, though her graphics links appear broken at the moment. Anyway, she made the phoenix and a bunch of wonderful other X-men icons.

If you want to know who any of my icons are by (a few of them are actually made by me) just click my userpic in the top right of my journal to see my userpic page which displays all the icons and credits for them in the comments.