Use imagemagic to put several JPG on one page
Solution 1:
You're looking for montage from ImageMagick :
NAME
montage - create a composite image by combining several sepa‐
rate images. The images are tiled on the composite image
optionally adorned with a border, frame, image name, and more.
If you already have convert then you almost certainly also have montage. To get a grid of 3x3 images per page on your pdf try this:
montage *.jpg -tile 3x3 assembly.pdf
This assumes your images all have roughly the same size. Have a read through the documentation for more complex options.