My pictures are normally 800x600 with a resolution of 180 pixels/inch. The billboards are 800x400, which translates to almost exactly 14 studs wide and about 7 studs high. I use a slide cutter to make sure the edges are straight, but even then you sort of have to eyeball it and trim to fit, so you might want to print a few copies in case you mess up cutting it out.
If you build a billboard like mine, the top and bottom pieces are 17 plates apart. They're listed as "plate, modified with door rail" on Bricklink:
http://www.bricklink.com/v2/catalog/catalogitem.page?P=4510http://www.bricklink.com/v2/catalog/catalogitem.page?P=32028I just used these types because it was what I had on hand, but I'm sure somebody can come up with a better build.
So to answer your question
>>8741878in theory you could download a high-resolution promo version of the image from Walmart or some other website, crop it to fit, and print it out at an equivalent resolution. I used glossy cardstock on a commercial office printer.
