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Of course there is a quality loss since JPEG is a "destructive" compression vs PNG which is lossless 
But it takes way too many space for photos with lossless compression.
You can either send PNG and let the site encode them to JPEG, or encode them yourself to JPEG: in that case, you should set 86% quality and enable interlacing (that's what the site uses). That quality may seem low, but it is for two reasons: reasonable disk space use, and maybe avoiding complain from copyright holders if images are too high quality (both are also a reason for not having full HD images).
If images submitted are not compressed enough, the site will re-compress them, inducing probably a little more quality loss.
When the picture had to be recompressed, it is mentioned in the picture upload window (in the green message).
If you send PNG there will be normally no more loss than if you encode them yourself to JPG, except if your program does it better than what the site does (using PHP's libs). I didn't make a comparison about that.
But it takes way too many space for photos with lossless compression.You can either send PNG and let the site encode them to JPEG, or encode them yourself to JPEG: in that case, you should set 86% quality and enable interlacing (that's what the site uses). That quality may seem low, but it is for two reasons: reasonable disk space use, and maybe avoiding complain from copyright holders if images are too high quality (both are also a reason for not having full HD images).
If images submitted are not compressed enough, the site will re-compress them, inducing probably a little more quality loss.
When the picture had to be recompressed, it is mentioned in the picture upload window (in the green message).
If you send PNG there will be normally no more loss than if you encode them yourself to JPG, except if your program does it better than what the site does (using PHP's libs). I didn't make a comparison about that.