Padre,

Whether or not you can have a postcard sized picture enlarged, retaining its quality, depends very much on the quality of the original postcard print. Generally for fine art prints these are pretty good. From 5 x 8 to 15 x 21 isn't that big a stretch, and you might get reasonable results. But you need to have someone (a professional someone) photograph the postcard, photographically enlarge it to the size you want and then digitally touch up the enlargement. It won't be perfect (and I believe the original was missing the burning faggots, of course), but it will be a reasonable representation; perhaps as good as the cheaper sort of art print. Seattle/Tacoma should have someone who can do this kind of work for you.

The picture really says it all, doesn't it?