I’m not quite sure what you mean, but if you are referring to an image that appears with your post snippit when your posts are listed on say your blog page, then this is called the featured image. You can set a featured image for any post by using the featured image module found in the right hand sidebar below Publish/Categories/Tags.
If you actually want to insert an image in your post then you would use the Add Media tab found above your wordpressing toolbar.
