In a number of forum questions over the years, I've seen devs struggling with using relative URIs to in-package resources. Typically it boisl down to the difference between something like "images/pix1.jpg" and "/images/pix1.jpg".
These two URIs mean, in the words of Jeremy Foster, "dive into the images folder and find pix1.jpg" and "go back to the root, then dive into the images folder and find pix1.jpg", respectively.
In Windows Store apps, it's helpful to remember that all such in-package 'relative' URIs are shorthands for ms-appx://<package_id>, as in ms-appx://<package_id>/images/pix1.jpg or just ms-appx:///images/pix1.jpg. A leading / on URIs, then, is what really makes the reference an absolute one, rather than a relative one. It just feels relative because you're not specifying a schema.
To be even more precise, a relative URI always must resolve against some base URI, which is defined by the referring document. Where people get tripped up is that when you're using WinJS page controls, such as ms-appx:///pages/somePage.html, then ms-appx:///pages becomes the base URI, and images/pix1.jpg will resolve to ms-appx:///pages/images/pix1.jpg. If you thought you were referring to the root images folder in the project, then you'll likely find the image not appearing at all. If you use /images/pix1.jpg, on the other hand, then you are making an absolute reference from the package root via shorthand, which resolves to ms-appx:///images/pix1.jpg.
The bottom line is that unless you really know that you're making a relative reference to the currently loaded HTML page, use a leading / to get back to your package root.