I've already got a folder of pics I use for profiles on various other sites. I'm leaning towards a cropped version of this.

That's really cool, that seems to go with your style nicely so it would acclimate well. I usually default to just using a cropped part of the linked image of Túrin from the Silmarillion. Sometimes I've used a Sutton Hoo helmet for a Beowulf/Old English vibe, but I'm not too sure if I'd change my picture for any real length of time. (Admittedly this is because I see lots of cool stuff I'd want to change it into, so I'd probably fall down a constantly changing profile picture rabbit hole if I let myself.)