Basically they are pictures that we have made or had made, and hosted on the internet, then we add it to our Signature in our profiles.
add the web address to your picture between: -
then enter the web address in the middle, then add at the end a [ then / then IMG then ].
I have written it like this above otherwise it would try and show as a picture.
Quite simple but effective, dont make it too big though :)