Where are the images for a .pkg / .dmg file located?
Images for most document types are provided by the application associated with that document type. For example, if you have TextEdit set as the default application for opening .txt files, the Finder will use an icon from TextEdit.app/Contents/Resources for those files (the exact icon file is controlled by TextEdit.app/Contents/Info.plist). But if you switch the default application to, say, BBEdit, then it'll switch to using the icon from BBEdit.app/Contents/Resources.

Installer packages and disk images work this way, so their icons are /System/Library/CoreServices/Installer.app/Contents/Resources/package.icns and /System/Library/CoreServices/DiskImageMounter.app/Contents/Resources/diskcopy-doc.icns respectively.