Probably I am very tired but I don't find the way to insert or edit the keywords to the uploaded images on Zazzle.
I mean, once I have uploaded an image if I double click on the thumbnail I can add title and description, but no keywords.
The only way to add keywords seems to create a product and then it is possible.
I am lost� 
I need your help!

It's Zazzle that's tired, it's always been like that.  As the image info does not populate the product info it's just for storage of your data. I put the keywords in the description field then cut and paste it in the product keywords.

I thought to do the same but I did not know that the image infos were only for personal use.
So yes it is a good trick.
Thank you.

(How in 2017 a site like this is not able to import IPTC data automatically? ?)