If the above solution did not work – try this – based on an old suggestion from Prasanna.
Create a new css class called no-image-border. Use a pair of divs in the html to enclose any image which needs no image border.
Put the following CSS in the graphene-child/style.css
.no-image-border .entry-content img, .no-image-border .child-page img {
border: none;
}
Insert opening and closing div commands in the html as follows
<div class="no-image-border">
normal html for image(s)
</div>