You can use any number of programs. If you just take it from a wallpaper, like Zubin suggests, you can just use something like Paint or whatever you want to use to crop pictures. If you want to make your own, programs like Photoshop or Frieworks work well, but they aren't free. Gimp is a program that you can get for free that's a bit more complex than your average photo editor so that could be a good choice.
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Appreciate what you've got because, basically, I'm fantastic.
gimp is horrible. dl a cracked photoshop jk dont. if u have windows, (or linux, idk if this works with it or not) get " Paint.net " its great, free, and even better than photoshop http://www.getpaint.net/
even if it takes fucking, 4 more seconds to start, it does make it worth $600 LESS than photoshop and when in the program it isnt as slow as you're explaining. idk what cpu you run, but it isn't slow for me.