I knew you could use light tables to put together collages and use print to pdf to output it. I then learnt that the book tool is a bit better since you can constrain layout a bit better; fit to 3x2 and align/scale images, etc - although it is a shame you can't have a one page book. But I still couldn't figure out getting a decent resolution image out:
Using Preview to open the pdf and save as a jpeg I got 144 ppi whatever resolution I actually picked. Which in this case meant an image of only 1701 × 1134 pixels when the collage was made up of images of 3000 x 2000 - so I really should have been able to get a higher resolution jpeg out.
In the book tool you can go straight to jpeg, but I still couldn't get anything higher than 144 ppi.
It didn't matter what (physical) size the pdf or book size was.
However, I then tried using Seascape and GraphicConverter (just because I still have an old copy of that around) to open the pdf file. In both you get to choose the resolution at which to open it and that way I was able to output a decent sized jpeg.