..."tilting" an image?

1) Not sure if correct forum.
2) "tilting" probably isn't the right word.

K so, lets say I have an image like this (this isn't mine I got it from an old old thread):

http://i46.tinypic.com/1t074g.png

How would I... "tilt" it so that it goes to the same angle as a normal poster? I saw people in this thread mentioning it http://forum.ragezone.com/f556/2-bad-customs-667936/ but still can't figure it out. It is probably such a silly question but I genuinely don't know.........

Oh I have GIMP + paint btw, so explaining on either of those would be fine.