Replacement Icons
How to replace icons
Address Book Icons
Drag into Quicksilver.app/Contents/Resources/ and replace existing icons. Backup Quicksilver first! Relaunch Quicksilver to see the changes. The .png files can safely replace the .icns files that are already present.
Other Icons
Drag into the Contents/Resources/ folder of the appropriate plugin. This is ~/Library/Application Support/Quicksilver/Plugins/QS iTunes Module.qsplugin/ in the case of the iTunes icons. Again, the .png files can safely replace the .icns files that are already present.
(Unfortunately, for some of the gorgeous icons below (the non-iTunes ones), it isn't at all clear what plugin(s) they can/should be used in.)
Replacement QS App Icon
Click to download! ~3MB. Two replacement icons included along with instructions. Supports the changing dock icons! Unfortunately this link is now non-functional.
Misc. Replacement Icons
Following icons by Ben Vallack Phototropic Productions
iTunes playlist icons
Set 1
Set 2
Contact Icons
Set 1
Set 2
Set3
iPhoto album Icons
Set 1 (standard dark drop shadow)
Set 2 (light glow around icon)
Currently unsupported iPhoto icons.
The current version of Quicksilver (Beta 24) does not support the use of these icons. I have put these icons here for when/if it does in the future. 
End of Ben Vallacks Icons.
Another Email Contact Icon
Like the original QS one - but 128 x 128 px. 
Tag Icons
Icons by Iconaholic.com
To replace the tag icon from the File Tagging Module plugin close Quicksilver. Go to ~/Library/Application Support/Quicksilver/PlugIns/ Right click on the File Tagging Module.qsplugin & select Show Package Contents. Go to Contents/Resources/ & replace Tag.png (make sure you rename the new image to Tag.png).
Multiple Selected Files Icon
To replace the Multiple Selected Files Icon you have to go to
/System/Library/Frameworks/AppKit.framework/Resources/
and there you will find NSMultipleFiles.tiff.
Make a tiff file of your preferred icon with 128×128 size and replace the NSMultipleFiles.tiff (after you make a backup of the original). To replace this file you must have admin rights.
