First, you need to know how to take a DOF screenshot. Go to your options and go to controls. In there is a place to bind the key(s) for taking a DOF screenshot.
Second, you'll have to get to the screenshot. If you go to "My Documents", look for the Blockland folder. In there should be a screenshots folder. Look in there to get the screenshot you took.
Third, search "Photobucket", this can help you with posting images on Blockland