"You can resize your images to a few key sizes, but it can quickly get out of hand " - This shouldn't be a problem. Just resize dynamically as needed and cache

"You can resize your images to a few key sizes, but it can quickly get out of hand " - This shouldn't be a problem. Just resize dynamically as needed and cache. Resizing on the server is a pretty fast operation, and you only need to do it once for any given form factor.