I've never approached it from that angle. I usually have some images to start with and I have to design some glue elements to make it look like the images are part of an overall theme. I use Paintshop Pro instead of Photoshop, but it doesn't matter which graphical editor you use as long as you understand how to use it.
The training videos at Lynda.com are a good way to learn how to use these tools. Here's some free videos on Microsoft's Expression Design to give you an idea of what these are like:
http://movielibrary.lynda.com/html/modPage.asp?ID=339
And here is Expression Blend, which is a tool especially for coders. This lets you animate graphics made with Expression Design:
http://movielibrary.lynda.com/html/modPage.asp?ID=384
The Expression tools are for WPF and Silverlight. They are vector-based and not bitmap based. I normally use bitmap tools like Paintshop Pro or Photoshop, since they are more appropriate for traditional ASP.NET. But Silverlight is coming on strong, and will be quite important moving forward.
Eric