I have two minimum image sizes to play with. One is 420x640px ("standing") and the other one is 980x420px ("landscape"). The normal loop with a regular layout is in place in my theme.
The goal is to produce a layout for a single post that first check the size of the featured image (ie. "check if post has featured image size max width of 420px, if true addclass layout-single-post-standing" or something like that).
I have been reading about responsive design but I am a beginner when it comes to that. Do any of you have an idea on where to start with this?
UPDATE: Dictating layout based on the size of a featured image could be considered a flimsy mechanism. As it panned out this method worked very well. By making use of the built-in twentyten_header_image_width feature (functions.php) and extending it with several conditional statements (wp_get_attachment_image_src and more) in the WP Loop, as well as setting several (add_image_)sizes, we managed to pull it off. It is a very complex WP Loop with tons of echos but it meet our goal of using featured image size to decide on what layout to present to the visitor.