Versperus said:
basically, the only time smaller resolution images will be accepted over the larger images is if the larger images were artificially upscaled and aren't the true size of the image.

https://e621.net/wiki/show/howto:sites_and_sources

This link teaches people how to get the best quality images from various sites, Taking the time to make sure that your upload is the best publicly available quality makes it very unlikely that your post would later be deleted for being inferior.

Well, if the higher resolution is a compressed JPG and zero-artifact PNG is only available in a slightly smaller format, I think we prefer the samller one. That's why I specified that the ones on Furaffinity are still JPG despite pretending to be PNG.