My understanding is that when a picture does not have a copyright watermark it is generally considered to be in the public domain (i.e. not copyrighted) and need not be licensed or have permission sought.Quote from atticus:
No attribution given. That's unethical and outright theft, but at least you're not a hypocrite!
http://www.elitetrader.com/vb/showthread.php?s=&postid=3104432&highlight=maverick74#post3104432
If my understanding is incorrect, let me know. Citing your source would be helpful.