I have taken over a WP7 project from another developer, it is my first experience of WP7 (although far from my first use of .NET and/or XAML).
I have just spent a fair bit of time shouting at an image for not rendering the way I expected (aka not rendering). I was setting it's source property in code behind.
I have finally discovered that because this particular image (the others rendered fine) was set as "Build Action = Resource" I had to reference it in a different way. Why on earth did it not exception, but that is not the question!
Can someone please explain the differences between "Build Action = Resource" and "Build Action = Content" with relation to Images. Please be specific, such as how does it effect performance (and why!) etc.