I've got them set as rotating desktop pictures, so one can "regenerate" into the next in proper order. There will eventually be at least one for each Doctor (maybe I'll give a shot at companions next), but as they've been in the works for well over a year, I figured waiting until all of them were complete might mean they're never actual seen by anyone else. More to come as they're done…
The obvious element to draw from for Four was his scarves. The more recognizable multi-colored above, and the later Tom Baker years below.
Five might be my favorite so far.
Six. A tartan within a tartan. I don't know which is uglier; the inspiration or the tartanized form. (Yes, that's a word.)
Seven. From the original series, the McCoy ones are probably the ones that I'm least familiar with. I'll update this if something doesn't strike me as quite right after I watch them again.