Whether you think of your visual or your headline first makes little difference. However, here are a few guidelines worth following. Though you don't absolutely require a visual, it will help draw attention to your ad. So if you use a visual, then you're already talking to twice as many people as you otherwise might. Another suggestion is to use photographs instead of illustrations whenever possible.
People tend to relate to realistic photographs more easily than unrealistic ones.
But whether you choose a photograph or an illustration, the most important criteria is that image be the most interesting one possible. The most important thing to remember here is that your headline must be short, snappy and must touch the people that read it. Your headline must affect the reader emotionally, either by making them laugh, making them angry, making them curious or making them think. If you can't think of a headline that does one of these four things, then keep thinking. However, with a little thought, a more underlying truth might be revealed - that Migrant Workers are as human as we are, and that our actions do hurt them.