The site of origin is the location (i.e. the physical folder) of the application executable assembly (i.e. the .exe that the user runs). The URI is thus relative to that folder.
Example: You have
The pack URIs are pack://siteoforigin:,,,/MyIcon.bmp
and pack://siteoforigin:,,,/Icons/MyOtherIcon.bmp
when running the MyApp.exe. That means you have to copy those files to those places.
In contrast, the application pack URI refers to resources embedded in your assembly (by using Build Action "Resource").