How big is the pdf that is eventually produced? Have you considered using thumbnails (i.e. smaller copies) of your big images? You could use GetThumbnail to produce smaller images optimized for your pdf layout.
They have a size of about 20 Mb, so it shouldn't be a problem.
Yes, it would be maybe faster with GetThumbnail, but I think that the resources should be enough, to do the pdf without GetThumbnail also fast.