If you have images for your sermon series, then you may want to use the new “Sermon Series” block and shortcode. That will display your sermon series images nicely. Clicking on an image takes the user to the sermons page with that series in view.
The images are best as 600x40px
I use the excellent canva.com to create most of our images. They have a lovely FREE offer for non-profits to use their service. On the site are hundreds of templates and images you can use. You can create designs of any size, with lots of preset sizes like Facebook header or Instagram post.