How can I set the mobile view of the site to maintain image aspect ratio?

I built my school's site on Grav and it seems great. My only problem is that when I view the pages from mobile at vertical view the images fit to the screen width but they don't keep the aspect ratio.
It also happens at my PC when I lessen the width of my browser window.

You can check here: https://gym-irakl.ser.sch.gr/site/el

Can I set it to keep the image aspect ratio?

Thank you. So, my problem was using the TinyMCE editor which doesn't support markdown commands. With a simple markdown image implementation the images became responsive.

Can I use markdown syntax in TinyMCE plugin?

@laviq,

Can I use markdown syntax in TinyMCE plugin?

Although slightly related to this topic, I would consider this to be a new topic. Would you mind creating a new topic with a suitable title?

Thank you again. Everything is fixed. I should prefer a markdown editor instead of TinyMCE and everything works perfectly now!