I found a workaround! I’m not so fluent in coding (I’m more of a designer) but basically, I inserted custom CSS into the head html and then inside each rectangle element, I linked back to the CSS with a div tag.
For those of you who are like me, insert this into the head html:
> <style>
>
> div.image {
> content:url(INSERTBASE64IMAGECODEHERE);
> width: 100%;
> }
> </style>
And then insert this tag into your element inner html:
> <div class="image" width="100%"></div>