Turn off your metro app by right clicking it and set it to turn off. Then if you don't want a particular app as the default to open such things, set your default programs to open specific things...eg photos, set photo viewer as default and the metro app wont open pictures. In my case I have Essentials 2012 photo viewer set as my default to open all pictures.
With a particular metro app open you can open the charms window by clicking the windows key + C and then click settings you can set the parameters/permissions of the metro app.
Wow...nice research.
I don't know what's happening with your system as I can hook up any camera or any other external device and photo viewer opens all mine....
Hold on I think I've got it....check your default programs...control panel, programs, default programs, set your default programs... then select your default program you want to open pictures and make sure it has all it default settings set.
As you can see in mine I have 21 of 74 defaults set. Open up the "choose defaults for this program" and uncheck all the extensions you see associated with the Photo's app. Just look for the icon and untick the box.
You don't need to do that for every photo. One is enough, then all the others follow automatically.It took me a long time to uncheck/change the defaults for all those Photo files.