Amnesty International's use of children in the following images, for ad campaigns about famine and child soldiers, is striking. This is made possible by the simple physical composition of these advertisements, as well as their locations.
This simple presentation helps you to focus immediately on what the children appear to be doing. The images are given just enough context to allow the viewer to speculate further about the circumstances within which the children are pictured. Because the illusion created makes them seem to appear in the physical space they've been placed in they are difficult to ignore, to say the least.