You could use the "old" Guvnor api to get this. At what point this may be deprecated I really don't know but works fine with Guvnor 5.3.1 which is used in jBPM 5.2. The URL for lets say my-image.png in snapshot LATEST would be:
http://localhost:8080/drools-guvnor/org.drools.guvnor.Guvnor/package/LATEST/my-image.png
You should be able to substitute LATEST with your snapshot name.
Hope this helps.