In a previous post, What Text is Appropriate, I talked about selecting appropriate text on Zazzle products where the text speaks in conjunction with the image and not for the image.
Another issue I seriously consider is what Zazzle products are appropriate for a particular photograph. Some photos may look good on a number of the offered products but jamming a photograph on a product that doesn't seem to fit the product shape makes no sense to me. In some cases, I have used some alternative methods.
I'm not saying some photographs, depending on what it is, don't look good on buttons or round magnets because I have many that do. Take the example below where I cut out the background and this guinea pig cutie and it fits on a lot more products.