The term poster child refers to a child representative of a class of people, for example a starving Ethiopian or a child suffering from some rare disease, whose picture is used on posters to raise money or awareness or to promote a cause. Such portrayals generally evoke pathos, and can be a very powerful rhetorical device.
In informal use, the term has come to be understood to mean a prominent representative of any class or group.
Shroud-waving is a distasteful extension of this device whereby campaigners use the death of someone to promote their agenda. Shroud-waving is the cause of bad legislation such as the dangerous dogs act, and is, like the poster child itself, a form of fallacious argument.
Troy Parker is the poster child for helmet promoters in the UK.