It wouldn't much help the download speed because the content, CSS, and normal images (ranks, buttons, etc) still have to be downloaded. Also, avatars have to be downloaded as well.
If the signature pictures are big in file size, then they'll, alone, take a little more time to download, but users can still browse the pages while they are downloading.
If the picture is big in screen size, then ask the user to reduce the size or replace the image with a smaller one. If the user won't change it, ask an admin and one of us will look in to asking them to change it or, if it comes to it, change it for them.
Happiness is not about being perfect.
It is about seeing beyond the imperfections.