The best way to do images in an HTML signature is by using absolute paths to images that exist on a server. If you include the actual images, many email clients treat them as attachments and the signature is mangled.

This hint isn't working for me when the images are referenced from a server as described above. It pastes into the signature area correctly, but when I create a new message, I can see the placeholder border for where images should be, but the images don't show up. When I receive an email with that signature from iOS, the placeholders don't even show up and the images aren't there.

This worked fine for me. I have a crafted signature that loads an image via webserver. Copying and pasting that from Mobile Mail to settings worked great. The image was shown while I was composing the email, and my Mac properly had to Load Images in order to view it.