It is very important to have your art and photography available on a variety of products. It doesn't have to be all the products. Your design doesn't even have to be on all the electronics cases available. But, at least work on getting your design on the most popular products.
In other words, if you're selling artwork only on an iPhone 3 cases when Apple's latest version is the iPhone 5 then you're not going to make money selling your designs at Zazzle. And when people don't make sales, they get discouraged and give up.
Purple Galaxy Cluster Case-Mate Case iPhone 5 Covers by annaleeblysse
Look at more Blue Casemate Cases at zazzle