[Theme: ToomMorel Lite] Set Featured Image not working

As other have said first up Theme is very pretty! Nice work. Ive got my home page, and now want to add an About page, with a nice large banner image. Im trying to set featured image (Ive tried all 5 page templates in the drop down) and though I can select and Set Featured image, and I can see its in my bottom right “Featured Image” box, when I publish/update the page there is no image displayed on the live site. Im guessing perhaps the banner image Im using may not be the correct size (im using an 800wide x 300 tall image). But I cant find notes anywhere on what correct size should be? Any help greatly appreciated.

https://www.ads-software.com/extend/themes/toommorel-lite/