If the png version of of an image is 1280x855 (or whatever the standard FA size is) and the jpeg version of the image is 4000x2000 then which one would be the better version to upload?

If it's notably bigger, and the compression artifacts aren't bad, the bigger version seems to be preferred. Be careful about FA's pngs, though. If the image is resized to the 1280px limit, it'll keep the png file extension but actually be a recompressed jpg. On some(?) browsers, if you open the direct image in its own tab, the tab's title might say what the image format really is.

SharkFetish said:
If the png version of of an image is 1280x855 (or whatever the standard FA size is) and the jpeg version of the image is 4000x2000 then which one would be the better version to upload?

with that change of resolution the twitter post, especially considering that FA 1280 is rarely actually png but the re-size and jpg filetype converted version of the png file the artist posted. Though in saying that, if twitter has 4k jpg and the file they wanted to post to FA was a png than there is probably a 4k png somewhere that if could be acquired would be the best version.

Official answer is which version has higher image quality, like watsit kinda said already.
e621:image_quality

Because higher resolution does increase the amount of detail, but at the same time JPG compression really commonly destroys detail which cannot be get back, even moreso than downscaling, so as long as resolution increase is high enough to justify the lost detail, then JPG is preferred.

Also if it's on the exact line wheter it's actually better or not, sometimes we do go with the dublicate case and keep initial upload. There are 10 source fields, so it should be easy to add in the other version as second source.

Thanks for the info guys.

Versperus said:
Though in saying that, if twitter has 4k jpg and the file they wanted to post to FA was a png than there is probably a 4k png somewhere that if could be acquired would be the best version.

I’ll see if I can find a larger png, but the 4k jpeg was also posted on FA. I don’t know if that changes anything.

SharkFetish said:
Thanks for the info guys.

I’ll see if I can find a larger png, but the 4k jpeg was also posted on FA. I don’t know if that changes anything.

If there is a 4k file on FA there is a good chance that it would be better than twitter, unless the uploader took the file from twitter. You can usually compare compression artifacting by looking into relatively colorful areas it can be visible to the eye.

If you have links to the FA and twitter we could do a comparison here if you're having trouble.

You can always try asking the artist if they have a full size PNG they're willing to share.