My reply is quite a technical one.
PNG is lossless. And generally not ideal for big photos published online because of file's size.
We use quite a small image size to display photos. Because of the site's layout.
There were cases when our images weighted about 1Mb after downsizing, which is inappropriate for smth. like 640x480, which we use. It's just too much.
Loads slow (not for everyone, but for many), eats tons of server's bandwith and storage. Not a surprise, though, it's png.
So we turned to JPEG here. And once your image is more or less good, the resulting
image will be good as well. Good _enough_ to be viewed online.
Well, here we have a comparison:
PNG compared to JPEG
So, that's it. We just can't post huge images to gain a bit (not a tons of) sharpness.
And while it's not a priority for now, we have smth. like "see raw image" feature
planned.