37signals lists 7 good reasons to avoid using Photoshop to create mockups for web sites or applications. I’ve found this practice utterly annoying for all the reasons that they mention but I don’t see it going away soon. Sadly most “web designers”, yes-that’s an intentional set of quotes, are much more fluent in creating Photoshop files than they are workign in CSS and HTML. Also, invariable, the designer throws in some widget, extra functionality, or weird, artsy border that is nigh on impossible to recreate in one browser, much less all major web browsers.
2 Photoshop gives you too many tools to focus on the details. When you use Photoshop you can’t help but pay attention to the details. The alignment, the specific colors, the exact shapes, the little details that may matter eventually but they certainly don’t matter now. The start is about the substance, not about the details. Details are for later.