Mobile version: Images not proportional

Hi

The mobile version of the website seems to change the proportions of the pictures. I tried to add this code in the CSS:

@media only screen and (max-width: 300px){
.img_thumb_feature {
width: 50% !important;
}
}

but it doesn’t work.
My website is https://www.success-inspired.co.uk/

thank you for your help