For those who leave the comfortable realm of Windows behind and enter the rather strange and initially maddening world of the Mac, the only thing that can possibly hit harder than not being able to use the keys and shortcuts like before, has to be the glaring lack of some MSPaint like utility.
Well, you may quickly find solace by going out and downloading either Gimp or Inkscape.
My personal vote would lean heavily towards Inkspace because of the abundance of excellent resources and tutorials when it comes to getting started with this tool:
screencasts are available as well.
Afterwards to get your creativity flowing use the following two awesome tutorials to create some mac/iPhone icons of your own: