How to include an image thumbnail in Categery view ?

I category view where you see just the post titles and their excerpts … what is the correct way to display an image for each post ? An image that designates the post – gives an expression of what is within the post.

I have noticed that an image will indeed appear when you include in the excerpt itself.

Is this the “correct way to display images” ?

Or is there a plugin tht let you select an image at post creation time ?