Well, too bad for y'all, because you don't get to ASK, I'm just going to TELL you.
This is the first userpic I uploaded. It's labelled "Froude number", because that's what Leper & I were saying when it was taken. Along with a large pile of other folks, we went to the scale model of the San Francisco Bay in Sausalito, and they had a sign talking about the Froude number, a dimensionless similarity number for objects moving through water. We took a picture because it's funny. Frooooooude. I use this icon for talking about science, or more often just for generally pontificating.
This is my "Mister Cranky-Pants" icon. I'm sure its uses are obvious. Mostly I'm just posting it because I know there are a number of you who are delighted by it, so I try to use it whenever it's warranted.
Chibi Dr. Tectonic is for talking about games.
This is a picture I took in a darkened bus, without flash, riding back from the airport. I included it as a lark in a set of pictures I sent to Jeb (50poundnote) as raw material for a bingo card, and he went and turned it into art. I use it when I'm talking about art, dreams, ineffability, and the like. (The text is Japanese kanji for 'dream'.)
In college, I lived at a phraternity. This is the logo we came up with for representing our haus. It's the 'xiohazard' logo, because it's xi chapter of the national. I recently added it for talking about teply things. I think that the actual graphic is mostly eto_theipi's doing. (And perhaps the design is, too; I am ancient and drooling and can't remember anything any more.)
Okay, that's five, that's enough for now.