Question.... If it is a free site why would you need to advertise? Is it something you are considering adding to another site? If so what about a tag along image? By that I mean after you resize an image. When the person goes to "save as" they get the one they have resized with your logo attached, along with a link that says" if you like your new image click here to get one without the text? Then you could either offer them the resize for a small cost? (may be prohibative due to the length of time required to use a credit card) or maybe redirect them to another site where you get credit from another URL for getting them more hits?
Just a thought,
Rufus