Skitch is my screenshot software of choice. Not only can you specify and take a snapshot, but you can then mark it up to hilight and point out relevant info, as well as draw mustaches. Then you can upload the results directly from Skitch or conveniently drag it to wherever you need it, be it a new email or your desktop.
Update: The company was bought out by Evernote. It’s now available for free from the Mac App store.