Replacing two images on main page for mobile only

Site URL: https://www.portraitsofwonderland.com/

I have two images on my main page which are quite large for standard viewing on a computer, but they need to be replaced for mobile only (simply sizing the original images will not be suitable for mobile). I have tried a couple of different suggestions based on what I could find from others, but I cannot get the images to be replaced correctly. I believe that they can be replaced through simple CSS change? Any suggestions would be greatly appreciated.

Add to Design > Custom CSS

/* Mobile Homepage resize images */
@media screen and (max-width:767px) {
[data-section-id="61ef6f753429967472e53935"] {
    min-height: unset !important;
    height: 25vh !important;
}
[data-section-id="62354c1913777e6bedda6592"] {
    min-height: unset !important;
    height: 55vh;
}
}