Hello Jena,
Thanks for asking. Hope you’re having a great day 
To adjust the image size in screen resolution 768px to 767px, you can take advantage of CSS media queries. First I suggest you to add a class name to both the elements under Customize tab. Here is a screencast that you can take a look. https://cl.ly/343X1I1O4140
After that use CSS media queries to adjust the width and look of image. For ex:
@media only screen and (min-width: 768px) and (max-width: 959px) {
.image-width-class{
` /* CSS codes goes here. */`
}
}
You need to replace image-width-class with your class name and / CSS codes goes here. / with the CSS code.
I am sharing few resources that you can take a look to learn more about media queries.
https://www.w3schools.com/css/css_rwd_mediaqueries.asp
https://www.w3schools.com/cssref/pr_dim_width.asp
Hope that helps. Have a Nice Weekend. 
Thanks.