As you have a mac the there's a graphics package called pixelmator which is very good, reasonably priced and allows you to slice up your design so that the images can be saved out separately. 

This is similar to adobe's fireworks which is coming end of life shortly as Adobe have annouced they are no longer going to be updating it.  Existing users of FW are expect to move on to Photoshop.

In the Free open source area there is Gimp as Mac mentioned which very popular as it's free.  Also there is another application called inkscape which is also feature rich for creating graphics for both Bitmap and Vector based.

Hope it helps.