Depends. If you consider that in each request a big chunk of time is spent one opening the connection, and that you can even start opening the connection to download the linked picture after you have received the response from the first hit, then maybe it's not a bad idea. It's one round trip worth of time that you shame off the total loading time.

However, if the image is very large, it will make the initial request large as well. I would only use this for images that are small and above the fold.