Is there a way to get the "Photos" app into the "open with" list in the context menu of jpg files as it is in the explorer of win 8?
Not currently. The way Metro apps add themselves to the Open With menu in the registry is undocumented, which makes it difficult to support them in menus which we build ourselves (vs places where we just show the system-generated menu). There is no known way to get the name + icon of the app (it's just a big, opaque ID number to us). We spent a while looking at it but didn't get far & have shelved it for now, since there is so little demand for using Metro apps. If things change we'll look into it again.
Thanks for the quick reply.
In this case it is a disadvantage to build own (context) menus.
What about giving the global choice DO context menus or Explorer context menus?