Child themes and framework

I’m new at wordpress. Currently I’m using a multisite for which I want to make child themes so that users can choose their own theme. I have heard that working with child themes is better for what I’m doing as apposed of making themes from scratch because child themes are easy to keep up to date. The functionality will be the same for each child theme but the CSS will be different on each theme. I’ve tried using the thematic framework for the parent themebut so far I can’t access the image folder to display the right images, so it’s kind of frustrating. Does anybody knows about a good framework that I can use for my parent theme assuming I keep on going with the idea of having a framework? Or can anybody point me to a good documentation for the thematic framework other than their website?