Change featured image in main blogpost

Hi thanx for the replies of my last posts, next thing: i read the threads of other people with the same problem but could not find the right answer. on the homepage with every post you place a featured image which is good but when opening this post you see that same featured image as a the top main image again. Is it possible to keep the small image at the home for what it is but when you open the post to change the big main picture to another picture?

example of featured image:
https://www.mupload.nl/img/0oel8c.37.30.png

example of blog on home which is good:
https://www.mupload.nl/img/4sz3q84.49.10.png

The big picture on top which i like to change:
https://www.mupload.nl/img/tljnhv9i2vf15.39.45.png

If anyone could help would be great.