Stretched Images on Viewer

[Erwinstein]

First of all, sorry for asking such a basic question, however, I really searched everywhere on the forum and Google and I can't find an answer to this.

The thing is, I started moving my e-book collection to Calibre.
Now, every book I open on the viewer, all of the images just scales with the viewer window. (Of course, if I double click the image, it will open on a new window with the aspect ratio preserved. That's a hassle though).

I tried checking all of my epub's markup on Sigil, but on the preview window of Sigil, things are okay (the image doesn't stretch). I know HTML and CSS though, so I was thinking that it could probably be fixed by editing it, but doing so would require me to edit every book.

I actually came from using Moon+ Reader on my phone for a couple of years, and these epubs all appeared alright on it, none of those files I tried had stretched images when opened on there, plus on a few other epub readers I tried just to check if what the problem was. It was just Calibre's viewer that has this problem and it is making me crazy. Even at Calibre content server, the same thing happens on its viewer.

I've been searching for a solution for days now. I don't know if my frustration was just making me miss a setting, but I literally read every setting I can find on the settings screen of Calibre and the reader, but I can't find something that work. But yes, I could have just missed it so if there is a setting for that, please point me to the right direction.

If not, can you suggest something that I may do? Copy-pasting a snippet of a CSS img selector on every book may be easy, but I currently have about 500 books migrated, and have more, so I would be glad if there's another way I can try to do. Thanks!

[kovidgoyal]

The viewer doesn't stretch images unless there is CSS in the book asking for the images to be stretched. You need to provide a sample, for us to be able to help you further.