when i posting, I have small problem

I know this is going to very basic question. But I tried entire day for this problem and Finally I’m fail.. I Really need you guy’s help
Above all, I use tanzaku’s theme ( https://www.tripleships.com), I don’t think this is theme’s problem

Anyway, Please check out my blog

https://musicfeeler.comeze.com/wordpress/

when i posting it, I have a problem.

As you see panda picture , I don’t wanna showing full size picture at the first page (Grid?) , which means when i write or post something, I just wanna show thumbnail or relatively small size picture.
when someone click the excerpt or picture, then i wanna show full size picture. Of course wanna do same thing to Video file

I’m sorry but I spend almost 5hr for this and I’m totally knock-out

I expect you guys good advise, thanks