I tend to browse image sites quite frequently. DeviantArt, or tumblr, had quite a huge feed when google still had their feedreader. When I see an image that speaks to me, I save it, and then when I need a new avatar I take my pick.
I've not noticed a trend in magicians, so I have nothing to add there.
To me, a good display image reminds me of the person who uses it. There are a few people here on E who change avatars every few weeks, but I know who it is before I read the name because their image feels like them. It doesn't have to be the use of colours or subject, it's the spirit of the image itself.