Some of them are too big to fit my screen

It's annoying

Or click "My Account" and then click "Settings"
Scroll down to "Resize images" and check it.

That will resize the images by default

While those are ways to resolve the issue, mobile devices should be considered in the code for that particular problem, on the mobile version of the site at least.

My phone automatically resized the page to make the image fit, I just have to zoom in if I want to read the comments but that takes only a double tap on a comment.

But yeah, different local settings, or simply mobile settings to override while on a phone, would be awesome.

Use the mobile version, I know it's kinda shitty where it doesn't let you favorite, or use basically any of the tools on the side, but it auto sizes for the phone.
But in all honesty, just go to settings and resize auto.