Zazzle Image GuidelinesRef:
Summary below or you can read the full reference if you have lots of time and patience to spare.
Support Zazzle Image Types
JPEG, TIFF, PNG, GIF,
Color Profile: sRGB (recommended) or CMYK
Here is Zazzle's Help on "Recommended Image Sizes and Resolutions"
As best as I can gather, one does not need to create a separate image of each product type. If your image is larger than the minimum specified, and is of equal or higher resolution, then you can use that image across different product types and resize image during creation.
Zazzle Help says "recommend you use images which are equal to or larger than the recommended sizes... "
Guide Files contains essential size-templates (for all the various product types) that you can download and use to create with. Available in PNG and PSD. Some also have AI format.
Take some time now to download these to your computer for all future use.
TIP: How adding Text will impact Image
The sample image guides will look something like this:
And you are to replace that with your image. Each image guide provides you with an accurate maximum size for various product types (e.g. t-shirt with horizontal image, shoes, etc)
But if you intent to have text (inserted when you create your product in Zazzle) either above or below your image and you don't want that text to be over-lapping your image, then you might want to make sure that the image you create and upload is smaller than the image-guide-size you use. The guide shows the MAXIMUM printing area for whatever product type you are creating the image for.