One way to do it (and I'm sure there are many) is to override the CSS by adding the following in your Before Fields section of the form.  You can use something like the following, adjusting the numbers to meet your needs.

.with_frm_style, .frm_forms {
    --image-size: 100px !important;
}
.frm_image_option_container img {
    width: 100px !important;
    height: 50px !important;
}

Thanks for the quick answer, rolevine,
 
I am afraid I am quite new in this environment. I went to Settings -> Customize HTML -> Before Fields.
 
Then I pasted the code at the end of the field. But it does not work (I send a screenshot). What am I doing wrong?
 
Thank you very much again
 
Best wishes