but I feel my knowledge of PHP itself isn't really up to doing this job.
Hum ... maybe I am up to it Chowny ... maybe I am.
Seriously, how about a dropdown list from your ACP - > Add / edit images so that you could specify - from each uploads - the size you'd upload ? From there, it would be far more easier for 4images to figure which size needs to be active from your details page.
Please advise.