i know that PNGs have less artifacts than JPGs, but when there is no visible difference between the JPG and PNG version of a picture, is it necessary to always give PNG the priority?

take into consideration the amount of disk space that is pointlessly wasted when thousands of poorly compressed PNGs are given priority over their JPG counterparts that look exactly the same.

PNG is a lossless compression algorithm, JPG is a lossy one.

If you save something as PNG it has no compression artifacts at all, whereas JPG will always have some artifacts even at 100% quality.

Thus, yes, the PNG is always superior to JPG even if you can't visually see a difference.

Berylium said:
i know that PNGs have less artifacts than JPGs, but when there is no visible difference between the JPG and PNG version of a picture, is it necessary to always give PNG the priority?

take into consideration the amount of disk space that is pointlessly wasted when thousands of poorly compressed PNGs are given priority over their JPG counterparts that look exactly the same.

Generation loss will happen with JPG files. It doesn't happen with PNG files. For the reason NotMeNotYou said above.
https://xkcd.com/1683/

Even if we do disregard that and assume content is always being directly uploaded for us, PNG does still ensure that the content is as it's supposed to be and it wasn't altered by any of the web services.

e621 has 100 MB filesize limit for the reason because we want all and every content as high quality as possible. By default nowdays for guests, the posts are served as JPG samples, so it doesn't matter if the post itself is PNG or JPG, so that user doesn't need to worry about files being too massive, for old users they can change this on their user settings.

If you are too worried about your local collection, I would strongly suggest againts conversion to JPG files and instead just using software like Pinga or PNGGauntlet to do maximum lossless compression and save on filesize.

e621:image_quality

Also I would argue that if you want lossy format so that there's no visual quality drop and can fit it in smaller space, then why JPG?
As with music, FLAC is for lossless storing, but if you want to transfer lossy music e.g. to your phone, then 128kbps AAC not only takes half the space but sounds as good if not better than 320kbps MP3.
JPG is designed for photographs, so if you want to save your favorite drawn furry images in lossy formats, WebP offers far better compression with less artifacting.

As twitter mobile actually supports this format, you can see the differences yourself with ease by changing the ?format= in the image URL. Let's take post #1421741 as example: PNG (447KB), JPG (113KB), WebP (72KB) and side-by-side.