Set the width as 1em in _idgenobjectattribute, deleted the height. It worked! The image looks fine now, it's tiny the way it's almost certainly supposed to be.
It did make everything else using _idgenobjectattribute tiny as well, but I can live with that. Thank you so much!
The CSS in the EPUB file;
<div class="_idGenObjectLayout-2">
<div id="_idContainer007">
<img alt="" class="_idGenObjectAttribute-1" role="presentation" src="image/shutterstock_81955303-2_vine2.png"/>
</div>
</div>
div._idGenObjectLayout-2 {
text-align: center;
}
img._idGenObjectAttribute-1 {
height: 100%;
min-width: 100%;
width: 100%;
}