c4d is a render made with the program cinema4d. these are usually
abstract renders, and people use them in sigs to get certain effects
and to make the sigs look more interesting. example of a c4d render:
if you search for c4d renders on
http://www.planetrenders.net, you will also
find many renders with a transparent background. this can make it
easier to implement the c4d into your sig. the c4d's with black or
white background can also be blended in but you should read some tuts
on that :]