Renders:
http://www.planetrenders.net/
http://www.animerender.com/ (I get most of my renders from here)
http://www.deviantart.com/
Try making signatures with sizes such as these and I suggest you follow tutorials, it's a good start :) As for your signature, the rocky texture matches with the render and not to mention the text blends with the background pretty well, Keep it up!