It is now fixed. Thank you!
I noticed after you had done something that my pictures were stretched oddly, but removing the custom CSS I had put in before:

.big-preview img {
    height: 60% !important;
    width: auto !important;
}

fixed that issue. Thank you for your help!

Nermit

Just making sure I didn’t confuse you with the link I included as private content before:

http://www.calujules.com/blog – here it looks OK (preview image size here set to Medium, 300×300)
https://calujules.com/news/tango-meets-flamenco-guanaquita/ – this one looks wrong on mobile (original image size).

Yes, it only occurs when I view it from an actual mobile device. I have tested it from more than one phone, and with different web browsers.
Kind regards,
Nermit