This is how i did it.
I select the no image setting and then add the image via HTML into the "Text for add link" and "Text for remove link" sections. This allows me to have different images for a favorited and non favorited post. You can also change the colour of the each doing it this way too.
Here's an example using Font Awesome Icons with the second link changing colour.
<i class="fa fa-plus-square"></i> Add to favorites
<p style="color:#428BCA"><i class="fa fa-minus-square"></i> Remove from favourites</p>