For example, here's a (scaled down) image I edited together in
GIMP for the first 9 cards from the Mini Set Design Contest (printed on legal sized paper):
Card images width x height in pixels: 696x1074
Page image size: 2060x3200
Coordinates for upper left corners of each card image:
(0, 0) (681, 0) (1362, 0)
(0, 1057) (681, 1057) (1362, 1057)
(0, 2114) (681, 2114) (1362, 2114)
This gives a Horiz 14 pixel overlap and a Vert 16 pixel overlap.
To get the cards printed out the right size in GIMP, use Print-->Image Settings and adjust preferences to
X Resolution = [width of card image in pixels]/59 = 11.8 pixels/mm
Y Resolution = [height of card image in pixels]/91 = 11.8 pixels/mm
(since Dominion cards are 59mm wide and 91mm high)
In my example above with images 696x1074, X and Y Resolution end up being ~11.8 pixels/mm.
Another option for editing is to use the montage operation of
Image Magick (command line utility):
https://www.imagemagick.org/script/montage.phpTo overlap card images in a 4x2 layout with Horiz 14 pixel overlap and Vert 16 pixel overlap:
magick montage *.jpg -tile 4x2 -geometry 696x1074-14-16 montage.png
This could then be printed out in Landscape mode on Letter sized paper.
You can adjust the parameters as needed, based on the image size of your cards (provided it is consistent from card to card).