There are many factors beyond screen size that could be involved in selection of image DPI: bandwidth availability and cost, screen density and current zoom level, amount of free memory available, hardware limitations (like iPad's max 1024px bitmaps).
Sending all these factors to the server would be problematic, so IMHO it's better if page declaratively specifies what sizes are available and lets UA pick one (so my UA doesn't need to tell every server when I'm out of my bandwidth allowance and hope server acts as expected).
I disagree to some extent. Given DPI, screen density and zoom level, the CSS etc should dictate the pixel width/height which can be sent to the server to obtain the correct version.
Sending all these factors to the server would be problematic, so IMHO it's better if page declaratively specifies what sizes are available and lets UA pick one (so my UA doesn't need to tell every server when I'm out of my bandwidth allowance and hope server acts as expected).