The easiest way to get images of all pages of a document is to export it as a HTML pageset. This creates .png images for all pages that can be post-processed to cut out the portions for the individual charts. One remark: for synthetic pictures, the png format yields better results than jpeg, the latter is optimized for photographs of natural scenes.
If you want to create single images from the charts, you have to loop over the pages of the document and for each page over the shapes. If the shapes DocObject is a Diagram2D (or Diagram3D) you can export it using the Export method mentioned in your posting.
