The ABScreenMaker is handy tool made by user Bab6 that allows you to take a picture of your gameplay at the required dimesions and and file type needed for sending screenshots to the MineMap or other maps. It looks like this:

Photo by Moneymaker1

It can be downloaded here. There is no installation required.  All you need to do to take a picture is press ALT+Space.

To get all the system requirements go here.