For anyone who does iOS development, I made a library for you! IMRemoteImageSize will allow you to determine the size of a JPG, GIF, PNG, or BMP image on the internet in milliseconds, without downloading the whole image.
It opens a network request to the URL you provide, downloads a few bytes, then cuts off the connection, and parses the bytes it receives to determine the type, and then the size of the image from the headers of that type.