Location of Fullscreen Icon in Mavericks
Using Pacifist and searching for "fullscreen-hover" in an Install OS X Mavericks.app bundle, I found fullscreen-hover.tiff located at:

/System/Library/Frameworks/Quartz.framework/Versions/A/Frameworks/QuickLookUI.framework/Versions/A/Resources/fullscreen-hover.tiff


Note that even thought I searched the installer bundle, nonetheless, the installed location should be the same.