http://z8.invisionfree.com/WCRemix/inde ... howforum=8
If you have to be a member to see, I recommend you join anyway, unless their not accepting new members (not sure if WCX is still not allowing new members).
Find a pic that looks cool, bring it into the sig, desaturate it, adjust it (Image > Adjustments), Filter it, find a good blending mode for it, and adjust its fill/opacity until it looks good. Experiment with everything in the menus (Image, Filter, etc).
My sig has a few images in it that I found off deviantart and some other place I don't remember. I think I changed one image's brightness/contrast with some blending mode (just select one blending mode and change it with the arrow up and down keys until you find something you like). If you find the effect is overdone, lower its opacity/fill, or just add a layer mask and brush with a soft black brush (I usually put the brushes opacity around 30 and click multiple times if some areas are more overdone than others, am I making any sense? lol). Definitely experiment with lighting effects, change the mode (I think that's what it's called), color, and all of the settings. Use it on applied images or just layers filled with white.
These tips weren't really organized well but I just wrote out whatever thoughts I had.
Ask me if you need me to explain any of these things further...