Most magazine-style themes draw their homepage and archive images from custom fields added to each post by the user as they’re written. This can be a time-consuming process, particularly if you’re using a plugin (Amazon Autoposter, for example) to create many posts automatically. It’s also a step that a lot of newcomers find confusing, and if you’re building websites to sell them you can find that even more of your time is eaten up in answering queries on the procedure.
In contrast, the Abundance theme uses a simple script to draw the first image from each post and then re-sizes it – a major time saver if you’re using something like Amazon Autoposter, because it means that you don’t have to open and edit every post in order to add a custom field. If the script can’t find an image in the post, it then looks for a custom field and uses that instead. If it finds nothing, it simply displays nothing.