Choosing the image which is preview before entering the post

Hello! I am using your theme and i really like it. The only issue that i am having is that i would like to determinate the Image that you see before entering each entrance on the blog. At first i thought that it was due to the order in which they appear on the entrance, or which they were uploaded, but it seems to be no logic on it as it seems to be random. Is this ok (should this work like this)? Or i am doing something wrong? Tried to use featured images to see if this works, but no and then i get two pictures just the same when opening the post. Just would like that the picture seen would be the first one that appears on each entrance. The web page is: , but i do not know if you can see anything from it. Thanks, Regards,