Rather than centering the images on the cards, move the images so that three of the four margins are the same.

For example, on the standard cards where the top margin looks about 1 inch, make the left margin the same 1 inch. Then you only have to cut the right margin, not both left and right to make the cards square. On the Large Cards, move the images up an inch or two so that only the bottom needs to be cut. (And maybe faint guide lines to know where to cut.)

This would make the cards neater and speed up the cutting process.