Hey Michael,
Thank you for your purchase and welcome to the premium plugin support forums! We are fairly limited on these types of support requests due to the fact it relates to your custom theme. I recommend reviewing our Featured Images Knowledgebase article for more information on how to go about adding/customizing featured images so they compliment the look and feel of your site.
The basic concept is the featured images you’re using should all be roughly the same width/height. You should use an image larger than or the same size as the space it is set to fill to avoid stretching, thus losing quality. You can add/remove/edit featured image sizes, usually on your theme’s functions.php file and call in the size you need, where you need them. You can also use CSS to set the dimensions that best suit your needs.
Sorry I can’t be of more help and best of luck with the featured images. For more info on what is covered on the support forums, please review our What support is provided for license holders? Knowledgebase article. Let me know if you’ve got any more questions and take care!