You need the Screen capture module. Following is an extract from http://mysite.verizon.net/hmelman/Quicksilver.pdf
Screen Capture The Screen Capture Module plug-in installs three Internal Commands that use Grab.app for screen captures. You’ll find them in the Catalog, under Quicksilver, under Internal Commands (make sure this source is enabled). You’ll need to have Advanced features enabled for them to work. They are called Capture Screen, Capture Region, and Capture Window and you use them with the Run action. You could just use the standard OS X key bindings for these. ⇧⌘3 for Capture Screen, ⇧⌘4 for Capture Region and ⇧⌘4 followed by space for Capture Window. The one advantage to the Quicksilver commands is that after they run, a new command window appears with the newly generated image selected as the object, ready for you to rename it or move it or do whatever you want. You could also get at these functions with the Services Module plug-in which makes things in the application’s Services menu into actions. This creates the actions Grab/Screen, Grab/ Selection, and Grab/Timed Screen which seem to work with any object. These also open a new command window but in B51 it doesn’t seem to work correctly. The object is listed as Un- known Clipboard Object and trying to Paste it didn’t work.
