Image on top of the article

Hello I would like to know if I get the Premium Theme I would be able to change the image from the top of my article. I don't know why but every time I post a new article on my blog, the image that was just supposed to put forward, it goes on top of the article when you open the article. How can I fixe it ? I have to get the Premieum Theme fisrt before being able to change it? Thank you