I know Ghostscript can convert PDFs to JPGs, and in the case of a multi-page PDF, can rip each page to an individual JPG. But is it possible to have it rip them to one JPG, so that the pages are pasted below each other, e.g. the top half of the JPG is page 1, the bottom half is page 2? Or do I have to use another program (and can ImageMagick do this?) to combine the JPG pages into one image?
Yes, you'll have to convert each PDF page into a single JPG file (Ghostscript can do that).
I'm not aware of any software which can do that in one go.
PDF-to-JPG conversion (with Ghostscript): You'll want to make sure that you get the best possible result. So make sure you tweak the commandline options so they work for you. I'd start with this:
This will create JPGs called pdffile-001.jpeg, pdffile-002.jpg etc. The parameter *-dJPEGQ=95" sets "JPEG Quality" to 95%. It uses a resolution of "600x600 dpi". You may need to additionally control the pagesize of the resulting JPGs in case your Ghostscript's default doesn't fit your needs:
You could use this command to stitch together 12 individual JPGs into one:
EDIT: (I had wanted to give this hint already in my original answer, but forgot.)
EDIT 2: (Missed still another important hint.) If you do not want to lose the good image quality during the stitching/montage process, which your PDF-to-JPG conversion had created, then also add the
Since ImageMagick has support for GhostScript built in, you can do the whole thing in one go:
which will take every page and create one long jpeg of them end-to-end.