Since version 2.9 WordPress has introduced a very useful way to manage images associated to posts and pages: the Post Thumbnails. It results particularly useful for theme developers, for now its very easy to display an image referring to the post content on the home page, in an archive page or in a widget.
The main informations on how to set and publish thumbnails can be found in the Function Reference of WordPress.org site. All you have to do is just to modify the functions.php file of the theme in use, adding a couple of lines of code. Mark Jaquith explains it clearly in his article titled New in WordPress 2.9: Post Thumbnail Images.
The best guide I found, anyway, has been published last year on WeFunction site by Liam (Beautiful Post Thumbnails: Top Examples & Best Practices). This article not only explains clearly how to write the lines of php code you need, but gives also many practical examples for a creative use of the post thumbnails. I recommend all the WP theme designers to take a look.
I recently made an experiment following the tips of the guides above (I did also check Practical Uses for the Post-Thumbnail Function in WordPress 2.9) to build a new italian-based site about ebooks, which is called ePubstore. With post thumbnails it has been very quick and easy to make a catalogue of ebooks linking each title to its cover. The way I chose to show archives is exactly the same as one I found suggested in the WeFunctions guide I mentioned above.