Thumbnail picture problem

[ Moderator note: moved to Fixing WordPress. ]

Hello!

I have a problem with thumbnail pictures. I am using the Cubic theme on my site. When I make a new post the program just randomly chooses whatever picture for thumbnail picture when I always want the first picture on the actual page to be the thumbnail picture. I don’t want to use the “Featured image” setting because it adds another disgusting picture on the page that shows up way too large that does not fit the design at all.

Is there any way to control the thumbnail picture so that it is always the first picture on the page that shows in the first page?

Here is the site:

https://tonislightingfactory.wordpress.com/

Thank you very much!

-Toni-