Hi there, and congrats! Your system is going to allow to accomplish so many wonderful things
Here are a few answers to your questions: first, rice paper and icing sheets are very different. Icing images are made with sugar and different starches (tapioca or potato, for example), and are able to meld into icing, whereas rice paper (or wafer paper) isn't; it'll just sit on the top of your cake and won't cut easily. Rice paper is great for making dimensional decorations like butterflies, but it's not great for printing, particularly photos.
New avenues for pre-printed images are popping up daily, but if you're interested in licensed images, the only company that legally produces licensed images is Lucks Food Decorating Company (lucks.com). They sell through DecoPac and Bakery Crafts, and if you go to their website, you can find vendors who sell their stuff to retail consumers. They also make a ton of patterns in sheets and strips such as the ones you mentioned (clothesline, zebra, and so on).
Several companies sell plain sheets that you can print on, and the size closest to a "border" scale would be the strip format, about 10" x 2.25", and Lucks carries edible ribbon that is about 1" wide (a few patterns available, but largely solid colors). I don't think other companies are featuring that size yet. Wilton also just came out with a bunch of punches and cutters that allow you to create borders with edible papers (theirs are called Sugar Sheets), that perform just like craft paper punches (which decorators have been using for years on fondant, gumpaste and icing sheets).
And finally, yes, your background should be white. Icing images will absorb the color from your cake, so chocolate or deeply tinted icings aren't ideal. Often you can get away with placing images on light pastel colors, but white or cream cheese work best. If you plan to work on chocolate or dark colors, adhere your image first to a plaque of fondant or white chocolate and then place on your cake.
I'm not sure I understand the question about shipping, but if you're shipping images that you've printed, make sure you have a supply of air-tight, sealable pouches. Many small stores resell edible images by placing them in ziploc baggies, which are not air-tight...by the time people buy them, they are sometimes brittle from air exposure. If this is the only route you can take, double bag them at least
Hope this helps!