Tutorial Description - Dreamweaver's Tracing Image feature is a great tool for organizing your site layout. You can comp out your site in say Fireworks or Freehand and then bring it into Dreamweaver to use as a guide to align tables and layers...really anything!
You can use images in either .jpg, .gif, or .png formats.
Tracing images do not get exported with the html. So you don't have to worry about the size of the file. For example, many of the png's I use for tracing images are upwards of 500k, depending on the project. Don't be concerned with that because none of that will affect your final HTML document.