I need a second article image

Hey folks!
I am still working on that blog using the ‘Simple Grid’ theme by dessign.net. It’s a grid style theme and every article has its on tile in which you can see the article image and the title, like this.

I want to get ride of the normal title design and instead put there own created jpeg files (a different one for each article).
what do i have to change in the front- and backend??

thanks a lot