Unfortunately options available via Theme Options - Homepage Post Options will work only if blog is used on front page. Personally I would go for category page for blog like I have in theme demo: https://colorlib.com/travelify/category/new-work/
It this case you will have to modify content-extensions.php file to use Blog Image Medium for category archive pages. Here is another discussion with code example: https://colorlibsupport.com/t/the-template-blog-medium-images/#post-3053