Stamps

I am trying to stamp a model that I created.  When I open up the stamp tab I don't see any stock stamps so can't say if the work like in the video.  I have tried uploading a couple of my own. The first is the one I want, it is a jpeg with a camouflage pattern. I resize it to cover the entire side of the Barrel model and then double click on it but nothing happens, after a while I did notice that the pattern showed up in some small random crevasses of the model.  I then tried a simpler stamp with less going on and could not get it to stamp anything.  Are stamps broken, am I missing something, or am I just doing something wrong and can't figure out what it is?  Can any Jpeg be used as a stamp or is there a requirement on what can be used as a stamp? For instance does a stamp have to be a single color?  Thanks, I can't wait to get more involved with the software.  Please let me know if I can help with any testing to get this to work.