Not supposed to use copyrighted images.
But everyone does it anyway...
as for referencing outside sources: There is no hard fast rule.
*IMPO* Its good form to show what you used, and is really what the comment section is perfect for...
I feel you should list the outside sources used when the outside source dominates the chop.
Like the cigarette chop in the prize contest today. Or basically anything tilak does.
When you take bits and pieces (even copyrighted images) and make your own creation, its really your prerogative if you want to show how much work you put into making that.
if you ever feel like somethings fishy.
It will bring up anywhere on the web a particular image is used.
If it doesn't find anything, that means it's unique