If your designer is making your assets in Photoshop than you're going to run into all sorts of issues especially once you start dealing in the varying possible resolutions etc, which you'll end up trying to counter with things like embedding the image files into ViewBox
etc. There's not as much flexibility in usability with that route.
My suggestion, would be unless it's really necessary to use Photoshop assets in your work, create your assets in Illustrator instead and use an export to Actual Pure XAML converter like Mike Swansons AI to XAML tool (which is quite handy) then you'll be dealing with real XAML and vectors so you can take those assets and do a lot more with them in regards to things like Transforms, Storyboards, Property changes, etc. Plus it will all be vectored art so you can play with sizes /shapes to your hearts content and not lose the crisp lines and resolution.
Hope this helps.