The compression option for PDF export does, as far as I know, not crop the images.
I know we had disussions in the past about cropping images, because if the cropping does not follow 16 pixel multiples, the cropping will force a recompression of the file (losing quality), because JPEG compression compresses 16x16 pixel blocks. So I had a suggestion for script that looks at the desired crop but crops slightly larger (to avoid recompression). Then the image retains it's quality, and the visible part can be cropped by the image frame.