PDF files tend to be generated by converting from other formats. It was never designed to be re-editable like other document formats.
I'm not saying there isn't something that will extract pictures (and otherwise edit) PDF files on the iPad; but I have not seen one to date.
It should be possible. I can copy pictures using Preview on the iMac, even though I can not edit the documents themselves. Though I am probably not getting the full resolution.
BTW. I tried PDF Expert, and like GoodReader it was not able to select and copy pictures. Someone should give Adobe Reader a try and see if it works.