Run your app, or even in designer - hit alt+PrintScreen (or just Printscreen) key (that copies the image of your current window to your clipboard). I usually use alt+PrintScreen since it just copies the currently active window and thats usually what Im looking for. Just PrintScreen copies your whole desktop image.
Then open up Paint/Photoshop, whatever and hit Paste (ctrl+v) in a new document. From there you can crop, edit whatever you want.