Can I first ask how an image of a US flag has helped them generate 'lots of revenue', and what proof you have that the image belongs to you?
Secondly, did you display a copyright notice on the image itself, or anywhere on the page on which it was displayed? If you did then whoever has used the image has contravened copyright law, if not then any legal claim is going to be far more difficult to substantiate.
In any case, it's only ever sensible to contact the person concerned and explain that you would either like the image to be removed, or a royalty fee to be paid for it's continued use (this can be a regular, pre-agreed amount or linked to attached revenue).
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