Nice article from a technical perspective; however, I would go a different way from a general web structure / design standpoint.

If your image is part of the theme, then I would argue that it is styling and not content. That being the case, I think it would be far cleaner to handle the image in css. Since your css file(s) would already be in the theme, the image would be a relative path and the issue goes away.

For images that are actual content, they would not change with the theme and should be stored in a separate folder.

Obviously this would not cover every scenario and your technique works perfectly when necessary, but as a general rule I always prefer keeping a clear distinction between style and content.