Every time I sign up for something on the internet I debate whether to use my real name. I go back and forth on whether anonymity on the internet is a good thing. And then if you don't use your name, picking a username is such a difficult choice....
In any case, on Isotropic, Goko, and on this forum I am going by Polk5440 (pronounced "Polk fifty-four forty" or "Polk" for short). This is a reference to the last strong pre-Civil War American President James K. Polk and his slogan "54'40" or fight" (his slogan for trying to stake claim to the Oregon Country for US expansion). Polk strongly supported US expansion and as President-elect presided over the annexation of Texas. As President he acquired part of the Oregon Country (settling for the 49th parallel rather than the 54th) and won the Mexican-American War which resulted in acquiring most of the rest of the western states. I like using this name when playing games of conquest -- it seems very fitting. And since Dominion is loosely thematically about having the most territory, I play under it here. Also, like others have mentioned, when I first got on the internet, I was under the impression numbers were required in usernames, so they seem normal to me now.
Related is the Avatar. The painting Napoleon Crossing the Alps by Jean-Louis David is related in theme to my username and is a painting I like a lot. The fact that it is used as the Tactician image on Iso is really cool, and as Tactician is one of my favorite cards, made it an easy pick.