Most of the time, when you post something, the image on the front page with the title is the first image you have in your post. But there is a way you can change this.
Say you have 3 images in your post like this:
All you need to do, is to put a header tag around the image you want to display at the front. something like < h1></ h1> without the spaces.
I'm not sure if this works if you have a header already before the image you want to display though...