It looks like the image being used is 920px by 400px – http://dev.ecrmarketing.com/sl1/wp-content/uploads/2016/05/Spanish-Level-1-Teacher-Resources-920×400.jpg
If you make changes to thumbnail sizes, I’d recommend using the Regenerate Thumbnails plugin right after. Download, install, and run the plugin and it should adjust your images automatically. – https://wordpress.org/plugins/regenerate-thumbnails/
This is a plugin that we all use for Images/Thumbnails in WordPress. It’s pretty good at fixing things like that.
Let me know if that works.