Great artwork can be ruined by bad design. Great design can make even bad artwork look good. If you’re a great painter but graphic design isn’t really your thing, then find someone who’s thing it is. A postcard is likely your first chance to make a good impression and if it looks professional, you look professional, likewise, the opposite holds true as well. If you’re not computer design savvy, find someone who is, perhaps you could work out a trade in favor.