As a designer you surely cannot be satisfied using other artists stock art yourself? Sharing credit for the finished peice feels like defeating the point of my hard work in the first place personally. I have been known to use minor elements such as blood drips etc but nothing of substantial impact to an over all image, i.e. wings, etc... I try to use my own original work.
I build my own original stock peices and I'm more than happy to admit I've used them more than once but as TinFish rightly pointed out I have developed new skills in addition to the fact that the time it has taken to build them has paid for itself later on when saving time on other briefs.
I'm more of an illustrator focusing on comic art etc rather than a graphics designer however as someone who does enjoy producing logos and working for clients as well as someone who is weeks away from starting a design course after many years of putting it off and getting by self taught I will say I have the greatest respect for some of the designers here and one in particular has inspired me to further my education so for anyone interested in this topic you really couldnt go any further for inspiration.
Oh and because its show and tell...
http://i15.photobucket.com/albums/a386/jonnywildcard/logo.jpg
http://i15.photobucket.com/albums/a386/jonnywildcard/Title-2.jpg