Almost everyone who has done some WP7 development has run into this problem and has eventually figured it out, but I thought I'd try to help people find the answer a bit faster.

The problem: Instead of seeing your icons on the appbar, you get something like the following:

The cause: This happens when the image file's build action is not set to Content (assuming you have already included the image file in your project and have set IconUri to the correct path and file). This is a very easy mistake to make because Visual Studio chooses a default build action of Resource when you add images to your project.

The solution: You can each file's build action to Content in Visual Studio's Properties window: