Before anyone runs off and grabs Pixelmator, see if Preview.app can do what you need. If you just need to highlight something in an image or add a note, it can do that.
If you need more, grab Pixelmator. It's an excellent program for basic image manipulation and drawing. If you need professional level tools, grab Paint Tool SAI, Photoshop, or GIMP as you need. Pixelmator will not be a good replacement for this programs.
I also use Captur (http://cambhlumbulunk.blogspot.com/p/captur.html) for taking screenshots since I can never remember the obscure OS X shortcuts.