Before starting... We do remember that using other people's work without permission is Wrong, right?
But say you do borrow something and transform it fundamentally (fair use) or you ask or pay for permission for this use or you create every fragment yourself from scratch. Anything that ends up on the internet is susceptible to challenge from those nasty trolls that extort money (which seems to seldom make it back to actual creators) for copyright or patent infringement. The courts are starting to wise up to the trolls - but who wants to go to court?
BoingBoing has a ridiculous example of this kind of trolling today: a magazine shaken down for use of An Ink Blot.
Not this one - this is a free image from i2Clipart.com
Seriously. You can get grief from using a splatter of ink - in this magazine's case perfectly legitimately. Luckily their cover designer kept good records of their image sources.
So be careful out there!