I've just taken over a busy WordPress site that's got ~800 posts so far.
The site's been around long enough that it was started up before post_thumbnails was available. They worked around that shortcoming with a custom field called 'Image' that held a value of the relative path to the image, e.g., "/wp-content/uploads/2012/11/image.jpg"
The theme is using Tim Thumb to make the different thumb sizes.
I'd love to get away from this and just use the post_thumbnails feature and set the sizes in functions.php and get rid of timthumb.php altogether.
I've searched, but haven't found a good way to make the switch. Any suggestions?