Hi Teri, sorry to be late in replying...I did read your post back then but didn't have time to post. I too have done rock painting and have all of Lin Wellford's books. Yes Jennie, I find the rock shape will have a say on what it will be painted to look like.
I use cheap brushes and acrylic paints. There is also "Patio Paint" for this and it's a little more thicker...like plastic.
For those who want to try she also has a site. http://www.linwellford.com/
There is also a rock painting club on Yahoo with loads of members that will help with any questions. http://groups.yahoo.com/group/rockpainting
also do a Google search for more and don't forget YouTube has videos. I do have some rocks in my picturetrail site.