In InDesign I just laid out the postcard files to repeat the black and white side but to print various images on the reverse side. Crop marks for bleeds are the most crucial part. Other than that, choosing my professional printer and paper specs was the hardest part. Ultimately I'm glad I went with a quality printer and didn't print them myself since we'll have these forever. The final product rendered a stack of postcards that covered all of our favorite [Austin-approved] engagement photos.