You can control which parts of the screen will appear in the snapshot using the check boxes in the 'Options' group. The options available are as follows:
If this box is checked then KSnapshot will bring the selected window to the front of the desktop before grabbing the image. This option has no effect if you are grabbing the whole desktop.
If this box is checked then KSnapshot will hide itself as soon as you press the 'Grab' button. It will remain hidden until the snapshot has been taken.
If this box is checked then KSnapshot will grab an image of the window which the mouse cursor is currently pointing to, if it is not checked the whole desktop is grabbed.