When i add a picture as the main picture of an article, it is displayed like this on the front page:
Is it possible to modify it so that there is a background behind the picture with the same width as the article?
There is no easy way around it. The best option here is to use bigger images instead. In your case shown above it would look much better if you would use more negative(blank, white) space around this shoe and it would look as a great featured image.
Other option is to wrap a div around featured image that you could style using CSS but that way you would have only one option for background color and you wouldn’t be able to change it from post to post like if you could do by editing these images before uploading.