You should never give your pictures away for free.
That resembles a buy out, you image is spread all over, and as you have noticed you may get a download or two, or even 20.
But that amount does not compare to putting an image in the trash or the feel free to copy bin.
By giving away you undermine yourself, the agency and other agencies.
Luring you into do it is a competitive advantage for the agency on behalf of you and other agencies.
Do not do it.
Also if the agency wants to do fancy free marketing, it can do it out of the commission it gets.
Thats the purpose of agencies, they get shares because they market.
Its their meaning of life.
Your meaning of life is to produce content.