My use case was this: a client with many sites is using RSS feeds to populate site content. Usually this is a no brainer. We can put together a recipe of Feeds and Imagecache but this is a little different. Instead of the main image being a separate entity in the RSS feed, it's coded right in to the body.
So, the first thing we do is create the feed.
With the feed in place, the picture is wherever it came in.
Using a preprocess function, we can intercept the node's content and pull out the image's URL. This will come in handy in the template file. Here's some code to pull the image URL and then generate the path by using the Imagecache External module: