When creating images it is important to consider many things

1. Make sure the image is yours, from your own imagination, or you have the rights to use that image for resale purposes. You CANNOT just choose an image from the internet and use it modified or unmodified without written consent of the ORINGIAL creator, who may or may not be the person who has loaded it to the web. There are many image sites that say you can use the images, but be careful to make sure you can use them for resale, and that the original artist is the one giving you the permission. Many sites like photobucket allow people to upload photos and images, however they cannot guarantee that it is the original artist loading the photo, as many people I know have found their personal photos from their personal sites have been downloaded by others and placed into photo sharing sites such as photobucket. Also consider the zazzle rules which will prohibit some other designs from being sold on zazzle.

2. Make sure the image does not include anything which could be trademarked or copyrighted. This would include a name of a movie or famous quote, names, nicknames or image of a celebrity or sports figure, brand names or logos, this can apply also in photographs is there is a business viewable in the background, or even a brand logo on a persons clothing. In addition you cannot sell images of people famous or not without their permission in writing.

3. Make sure your images are a proper size to fit on products. You can use the zazzle guidefiles or Zazzle image sizes to see which sizes are needed for specific products, I generally use a 3000x3000 pixel 300dpi image for most of my products.