Post thumbnail image in search results page

Hey guys,
I was just updating my search.php when I noticed that the results are appearing as entire posts just like on the main page.
I thought it’ll look better if a post shows just the title, category and comments and a thumbnail image.
The image should be connected to the content of the post but I don’t want it to appear in the content (the single.php).
So how manually put a thumbnail image for each post and that thumbnail image to be used and to appear only when the post is a search result?
Hope you get what I mean.