Russel wrote a blog post about this, and you can find it here:
No it's actually very easy, you can add a lot more images as following:
Add an image collection in you project by "Add", "Page resources", "Image collection". In this collection you import the images you want to use. Second you go to the button, select "image" instead of glyphicon and you can select your new imported image there.
There are more ways to achieve custom images through theming and css. But this is the easiest way.