Divide any image into nine equal parts, and the majority of great ones will have their focal point at the hotspots. This is where the grid-lines cross, or where I've placed stars on the photograph above.
The way that Zazzle works is to assume that this will be true of your picture too. For most people, it will work perfectly, when the fill tool protects that over all else; or when the default alignment shows only that central square and its immediate environment.
However, my image is an example of where this whole basic tenet falls down. Arguably, the middle is the most boring element of the scene. The dark stream on the right is fundamental to the composition.
Fortunately, Zazzle provides us with the arrow tool for this kind of fine-tuning. This works best with things like ceramics or decorations, where only a small area of the image is used. That will always be the hotspot area.
Clicking up, down, right, left, as appropriate, you can re-align the picture to your chosen spot. But if you have one like mine, it may take some time to do this with dozens of products. If speed and efficiency is your thing, then it could be worth being more selective with the items that you choose for market; and only using images with traditional hotspot focal points on Zazzle.
With much thanks to Ember, who explained this facet of art photography to me!