We made a logo for our robot this year.
http://i.imgur.com/cC07pnQ.png?1
I used Photoshop and illustrator to make it.
I see others have recommended free photo editing programs.
My recommendation is to get a team Creative cloud subscription and then give select people the credentials. There is an academic discount and after that it becomes very affordable. Its definitely worth it to have the right programs to do the work instead of constantly having to find free alternatives.