Hi Andrew,
This is a tricky one…In looking at the plugin code and the HTML output generated by the plugin, there really is no difference between a featured image and a custom image. The featured image is displayed using a builtin WordPress function that adds a few extra CSS classes to the image as well as the height and width, but that is about it. My plugin does not attach any styling to these additional classes, but I am wondering if your plugin is doing so. Here is a comparison of the output…
Featured Image
<img width=“xxx" height=“xxx" src="https://image_url" class="attachment-full wp-post-image" alt="image_alt_text">
Custom Image
<img src=“image_url" alt=“image_alt_text”>
Is your site live? If so, can you try adding a featured image and then send me a link to the page. I am interested to see if by looking at the source, I can see if the image is even being added to the page. If it is, then it may be a styling issue.
Thanks,
Nick