If the project manager is the designer then understanding both roles is all the more important because the design often gets into small details of pixels in graphics. Yes pixels in graphics are important but the project manager knows that a pixel is not what will make a visitor into a client.
The project manager needs to set the time frame and keep to it. He needs to look at the project as a whole; all aspects of the web site. He needs to pay attention to getting the message out and the message is rarely about a few pixels.
Sometimes the project manager needs to say; lets get back to this image and take care of the pixels later because it is important in the bigger picture for the information to get posted.
To use a metaphor the project manager needs to work like a general not a foot soldier. His role is important, without a general the site will get stuck in the mud without supplies. If the designer is the project manager he needs to schedule his project management time and when his role is a project manager that role needs to be seen as the most important. He needs to delegate what needs to be worked on and where human resource time is needed for the bigger picture – even if he is only delegating to himself.