A close image keyboard shortcut is potentially dangerous in my opinion, and unnecessary given the options available. Besides FILE > Close and the X button on each window:
From the main menu:
FILE > Close All
FILE > Close All in the Current Workspace
From the Console window:
close <image-id-list>
where <image-id-list> is a set of image identifiers, which can include the wildcard characters (*?). For example:
close mask psf M31?_test
close *
close M81*
close --force <image-id-list>
Same as above but closes images unconditionally, that is, without asking for confirmation if the images have been modified and not saved (so be careful when using this option).
For more information you can use the help command:
help close
or just 'help' for information on console commands.