Open up your paint program and find the diablo screenshot
Next, move go to the right corner of the screenshot - press and hold a little square, then drag it
Example:
http://img807.imageshack.us/img807/6439/48102348.jpgOnce you do that, drag it to a reasonable distance (a white, extended field should appear)
You can crop out anything you want from the screenshot by selecting the rectangular marquee tool and putting it over the area you want cropped out.
Once you have that, press ctrl + c, then select the entire picture with the tool and press delete. After that, press ctrl + v and the cropped out picture should appear.
If you want to add another screenshot to the white canvas - open up the paint program again with the second screenshot (crop out whatever you want) and press ctrl+a and ctrl + c, then go to your paint window with the first screenshot and press ctrl + V