Just tried to add a Pin to my “Home and Beauty” board on Pinterest and received:
Had a look at the image I was trying to pin and its resolution was 2282×2282 pixels making it an approximately 1.2MB file.
Downscaled the image to 320×320 and it worked fine. Also tested another image on the page and discovered that a 1280×1280 image would pin without an error.
So in conclusion I’m not sure of exactly what is the maximum image that Pinterest will scrape and convert from a website site but it’s somewhere between 1280 x 1280 and under 2282 x 2282