Hi, Norki, and welcome to WetCanvas!
I don't think seeing a photo of a Zazzle print will be helpful, at all. There are too many variables that will determine what you see in that photo -- including the settings of the camera taking the photo, the skill of the photographer, and the calibration of the monitor that photo will be viewed on.
Ordering a sample print (or two) would be a much more reliable way to see the actual quality of a Zazzle print.
I can tell you that Zazzle has long been one of my favorite online print services, consistently producing very high-quality, fade-resistant prints. Currently, I'm maintaining four Zazzle stores and have exhibited and sold Zazzle prints, many times, including in gallery exhibitions.
Most of my large-format work is printed by Zazzle.
With every print service I've used, I've found that I need to lighten my images a bit before printing. This holds true no matter how carefully my monitor is calibrated.
CafePress was a major headache for me; I won't try them again. I've never tried Society6, but am thinking about it. Zazzle is definitely the best of the several that you mentioned, imo. Another great option is FineArtAmerica.com.
But, as I said before, actually ordering prints from any online print service is the best way to feel truly confident about the product received by your buyers.
Best of luck to you!