I think it depends not only on your coding ability but also on the type of design you are aiming for. If images are fairly important to your design then these need to be designed outside the browser, and moving back and forth between the two is probably slower. For example, perhaps you want a nice masthead photo image that is somehow integrated or overlaps with other elements on the page. During the design process you may want to play around with that image. You can’t do that in the browser.