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:

http://www.covenantalthoughts.com/wp-content/uploads/2014/03/number_one_2050.gif
<h1>http://bedsandall.co.za/wp-content/uploads/2015/09/Question-mark.jpg</h1>
http://worldartsme.com/images/three-clipart-1.jpg
<br><br>
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...