Yes, handling it on the server side sounds like a reasonable workaround. For example photo.flif?res=1x returns Content-Length: 100000, and photo.flif?res=2x returns Content-Length: 200000, with both returning data from the same file.

I say net loss because with current http the loss of closing the connection is typically much higher than the loss of downloading more bytes than you need. Connection reestablishment needs a lot of round trips.