Adding thumbnails to the front page of my Revolution Church theme?

Hello,

I having a hard time trying to figure out how to add the small thumbnails to the posts on the front page of my blog. No matter what I do or what plugin i install, the thumbnails always stay the same with the black and grey letter R on it. Could someone please help me out with this? I’d be very grateful.

Thank you

P.S. I’m using the Revolution Church theme and my site is https://www.hiphopcreed.com