For a different project I bought card blanks and printed the images out on paper.
I used UHU stic glue to attach the paper to the card. It's a bit like Pritt, but less lumpy. I would have thought hot glue would be very messy. You also need to watch out for glue warping the cards (eg pva). The UHU stic is "dry" so doesn't really warp anything (although I left the cards under a pile of books overnight, just in case)