First. Put just one render in the sig.
Second. don't put other smaller stocks ( images ) in a sig.. unless you make sure the edges fit/are invisible with the bg.
third.Make sure you use Renders for making a signature. ( example :
forth.try playing with some effects * such as filters/distort/ocean ripple ( or any of these there ) and not only with filter/disort but try all the stuff has there.
fifth.You must try some tutorials.