In order to sell on zazzle you need to understand what is legal and what is not.
what is legal? Designs/slogans you create from your own imagination using your own skills. You do not need to be an artist, but you need to be sure to follow the rules.
It is illegal to take an image, slogan, phrase or idea from the internet, tv, movie, book or anywhere else without getting express written consent from the original author/artist granting you permission not only to reproduce their work, but also to sell and profit from it.
This also applies to images of celebrities or famous people and other stars, whether they are from TV, moves, music, sports or otherwise. There is only 1 exception to the no famous people rule, politicians may be used ONLY in political context, and ONLY approved images released for public domain.
Sometimes there are sites that will sell images for artists to use, commonly called clipart. This is the type of images that come with your computer, are included in programs like MS word, or photoshop. They may also be purchased on disks or online. These may or may not be legal. The ones that come with MS word for example are NOT legal to use.
There are some images and books that are public domain, they can be used. However you must first verify that they truly are public domain. Do NOT accept ANY websites which have wiki in their name/url as an acceptable resource. A wiki site means anyone whether they truly know the truth or not can update and add information to their site. These site are often filled with half truths or misinformation. It is not always intentional, some people are ignorant of the laws and believe they are correct, when legally they are not.