Travis' apartment is uber-depressing for all sorts of reasons, but we have to say his artwork choice is one of the bleakest aspects of his whole décor scheme. He has a poster in his room saying, "One of these days I'm gonna get organ-iz-ized!" with the letters of "organiz-iz-ed" falling off a ledge. Underneath it, he has a "We the People" Charles Palantine poster.
C'mon, Travis. We just know that a nice van Gogh print would cheer you up… or would it make you cut your ear off? (There's no telling with this guy.)
Let's get back to the posters at hand: Travis does want to get organiz(iz)ed—but he ends up doing it in a basically crazy, mentally un-organized way. He starts getting in shape, training his body—but he's preparing himself for a rampage. He's becoming organized in one sense—curls for girls, amirite?—but in another, he's internally falling into disorder.
The Palantine poster seems to be a little ironic. Originally, Travis might have put it up because he really did support Palantine (albeit just because of Betsy). Later in the movie, though, it seems to be there because it represents something he hates. He might be using it to motivate his rage.