Images can often pose a major problem when considering a goal of attempting to convert the PDF back to the same level of quality as the original document. In many cases the PDF was generated where images have been down-sampled to a lower resolution, thus making it physically impossible to reach the goal of a flawless, exact duplicate of the original document.
The example shown is a PDF file created using QuarkXPress where the original placed images were 305dpi. Two PDF files were created out of QuarkXPress one using the Press Quality settings and one using the Low Quality settings. As you can see PDF2DTP can only extract the quality of image that was originally set when the PDF file was created.