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.

You can check it out on my GitHub.