Absolutely, you can integrate visual design and usability testing into Agile SD practices. Indeed visual design is naturally iterative when done right. The expert in this area is Jeff Patton, and I encourage you to read his blog. There is also a great interview about UX design and Agile design between Jared Spool and Jeff Patton here.
Basically, just as your developers build and prototype functionality, show it to the customer, and then iterate over it to gradually refine it, so too should your designers be part of that process. In our shop, we have the designer take part in almost every planning meeting, so that designs can be run by the engineers who need to make it happen. Frequently, our designers and programmers work together to refine the UI as we head toward launch. We're doing that for a site today, in fact, and it's going very well.
Hope that helps.