Add image to each post!

Hi all!

So, I’m developing this theme where every post will have a “signature” image. The idea is that the user should choose an image for each post it publishes and it will be displayed together with the post in the list.

My problem though is how to do this properly. I know that I could use a custom field and let the user insert the URL to an image. The problem with this though is that my user pretty much doesn’t know what a URL is. ?? And there is now way of extracting the width and height of the image this way…

So, I wonder if there is a plugin/templatesnippet that makes it possible to add a image from the media library from each post? So that you get all the relavent metadata about the image(url, widht, height and so forth) and can place in the HTML correctly not depending on where you insert the post itself. I’m thinking something like :
<?php postimage($postid); ?>

Cheers!