I have a bunch of photos I want to print at the local costco. Many of them are not the standard 4x6, 5x7 aspect ratio.

When I try to print them from the costco website, it insists on cropping the pictures to fill the paper.

This is not what I want. I want to fill at much of the paper as possible and leave the rest white.

How can I resize the files to create new image files with the proper ratio?

I'm using lightroom, but I'm fine with running some other utility.