To view Collections in my store Rainbowartistzanne on Zazzle US, do go to
https://www.zazzle.com/store/rainbowartistzanne/collections?rf=238177356453190171
Instagram: rainbowartistzanne
So, my process is to take a photo from my travels or a painting i have created; then place it in a filter app like Snapseed to enhance the brightness of the image. After that, i use Canva.com to resize the photo, depending on if i prefer the image to be horizontal (example: blank Presentation template) or vertical (eg: blank Poster template).
However, do read on how to enable Customization for your future merchandise on Zazzle's website.
https://help.zazzle.com/hc/en-us/articles/219543148-Product-Customization-Explained
As for Zazzle customers, it is interesting to choose from the morphing mugs which are a delightful gift. It looks like a plain mug, but once hot water is added, the image "magically" appears. This is actually heat sensitive coating technology. Some popular items are: iPhone cases, iPad cases and invitation cards. I like reading more about what other Zazzle Designers are selling or what have been their bestseller items.
Good luck in your endeavors. May post more articles in future.
Below are a few photos of merchandise / products designed. Enjoy!