A recent discussion with a new user prompted me to post this tip.
To quickly and easily unselect a raster from the Raster Manager, choose one of these methods:
1) If the Raster Manager is open, just click in the file list as indicated in the graphic below.
2) Use the Element Selection tool and click anywhere on the screen, being careful not to click on an element.
3) Use the following key in: IMGMANAGER ACTIVEIMAGE NONE . You could put it on a function key or even a custom button.