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.

Hello! This is my first time using this forum. You see, I've noticed that the only way to upload images to imcdb is through jpg files. In my latest uploads, I've uploaded PNG images that have been automatically converted to JPG. Is it advisable to download JPG images directly, or does it not matter? I ask because I've noticed a noticeable loss of quality from PNG to JPG images.

Minor update, following a request received, now in the page that appears when one uploads a picture for comments you can simply press the Enter key to copy that URL to clipboard (in addition to Ctrl+C or the "Copy" button at is side).

I suspect issues with some ads (I really should change the ad company), even if not optimized on mobile it should work fine, as it is rather basic pages.
This site is a relic from the 2000-2010s internet It was a little tweaked to make it more usable on mobile, but it is not a mobile-first site like most of the sites nowadays.

Just wanted to chime in and say that the mobile site is very unoptimized on Safari on iOS. Sometimes I can't click on links or even the text box if I need to edit a comment.

Small update for the popup of comment images: if a timetag can be guessed from the picture name, it is displayed. There may be cases where it is wrong in some cases, or not detected where it should, I can try to tweak the code if needed.

You can check in the thumbnails I post, e.g. in https://www.imcdb.org/yourcomments.php?formovie=33550045

I test mostly with Firefox and I didn't had that problem. Maybe it is due to some ads? You could try blocking them just to check if it is caused by that (as they depend on the country, maybe you get some that cause issues)

It happens with both but it's far more noticeable in vertical mode.

I use Firefox.

Do you mean that it happens when you are in vertical mode or in horizontal mode?
Which web browser do you use?

I've noticed a strange bug on mobile. For some reason, i can no longer click on anything on the right half of the page unless i flip my phone sideways. This only happens on this website specifically.