All our WordPress Themes use thumbnails to load images much faster. Thanks to thumbnails images loaded everywhere on your website e.g. on the listing card are much smaller than the original and the website is faster.
Secondly, images dedicated for mobile are also much smaller so the overall speed on the phone is increased. Fast mobile is one of the factors to have better Google search positions. Turning off thumbnails will break your website SEO ranking and visitors will be less satisfied because the website will work much slower. It is technically not possible to turn thumbnails off because of these reasons.
Here you can read how to regenerate thumbnails on your website.