seeing as the sensible responses come in this forum, I’ll post this one here.
I primarily use PNG for POD services. Unfortunately for me, difference services like different specs. For me, 300 dpi is always the base setting followed by an x/y pixel or inches dimension set. Redbubble is very forgiving. Zazzle not so much.
I noticed that for posters, if I upload a 300 dpi, 11" x 17" png image, the Zazzle sales page defaults to the price for a 11" x 17" product. Or larger if the parent file is set to higher x/y dimensions. So I uploaded 300 dpi by 8" x 10" or 8" x 12" files to keep the published price point attractive.
But, png files will enlarge within reason but at some point they deform. For now, I’m sticking with file dimensions falling within the recommended standard poster sizes. How? in your graphics program, uncheck any auto-adjusts. Manually enter the x/y dimensions while maintaining 300 dpi. Annoying? Yes.
What do you do?