At design time pack uri is valid, but not at runtime?

I'm setting a Button's content to an Image. It looks something like this:

<Button>
   <Image Source="pack://application:,,,/NavigationImages/nav_up_left.png" />
</Button>

In my project I have a subfolder named NavigationImages and within that folder is the image file nav_up_left.png.

When I view the Designer the image appears, however during runtime I get an IOException error saying it cannot locate the resource.

The Build Action is set to Resource.

Actually, this worked fine in one project. But when I copied it over the another project it fails. This seems like an incredibly simple problem, but I find myself stumped and ready to start pulling out hair. @_@

Your thoughts and suggestions will be much appreciated!