As I started using WPF at higher DPI resolutions, I got into troubles with scaling of toolbar images. XAML images solve that problem.
However I find it very hard to find free toolbar/ribbon XAML images that you can use in your own projects. What I found is usually not free and one probably needs more than few collections to get all needed images.
Where do you find your toolbar/ribbon .xaml images?
Check out the following link http://stackoverflow.com/questions/1124769/hidden-features-of-wpf-and-xaml/1405000#1405000
There is a list of converters that can take many formats of images and convert them to xaml for you.
Personally, I wind up using InkScape to create my XAML a lot. Even with my meager drawing skills, it's fairly easy to use and very effective.
As a note, though, I tend to work with the image in the native format and then save as XAML. Working with a file in XAML format seems to be somewhat unstable.