After being inactive for what seemed like forever, Brian Bolland's website is back online. Other than being able to view all his wonderful cover art, he has a 'Lessons' link which shares his process on creating a cover. The most interesting is the Gotham Knights cover, where he goes in depth about his choices and techniques. He mentions using Photoshop 5.5 so I'm assuming this is a fairly old article, but it's still very educational. Check it out.