You can add a “img-select” class attribute to field, and use this CSS :
.acf-field.img-select .acf-checkbox-list input,
.acf-field.img-select .acf-radio-list input{
position:absolute;
left:-9999px;
}
.acf-field.img-select .acf-checkbox-list img,
.acf-field.img-select .acf-radio-list img {
border: 2px solid transparent;
}
.acf-field.img-select .acf-checkbox-list input:checked + img,
.acf-field.img-select .acf-radio-list input:checked + img {
border: 2px solid #444;
}
😉