There has been a bit of discussion of the required 'passport photos' in this thread. I generated my own, as have a few others.
Get a good, clear picture against a light-colored featureless background. Follow all of the other guidelines for a passport photo; don't smile, no hat, etc.
I edited mine into a square format right in the photo app. I also lightened the background a little, but if you stand in front of a light enough wall to begin with, that isn't necessary. Leave enough background around you to allow for further cropping, and be sure to save high-resolution images at each step.
I used Preview to edit the size to 1.5" square; GraphicConverter or most any other image editor will also work, either by letting you specify the size, or by putting rulers on screen so you can drag to the size you want. This may not have been a necessary step.
I couldn't find any way to use the Walmart photo app to print one or more 1.5" images on a larger print, but if you use the Walgreens photo app, and select a collage print, 4" x 4" size, and select the collage layout that allows you to insert 4 same-size photos on the same print, it works great. When I picked up the print, each of the four images was 1.8" square, so I probably could have used the larger, square photos (skipping the edit to 1.5" sq) and they would have scaled them down to that same size. Simple enough to cut them down to 1.5" when cutting them from the collage print.
My all-in cost for four 1.5" passport photos? 53 cents.