Chances are that with your current privacy settings, you’re allowing Facebook and it’s partners to use YOUR PHOTOS in their advertising material, i.e. some of the ads you see in the right column of Facebook.
There’s two things you can do about it, unless you don’t care:
1) Add a watermark to all your pictures, then hopefully the advertisers/Facebook don’t want to use them – or you get free advertising for whatever watermark you put in.
2) Deactivate the “Appearance in Facebook Ads” function. To do this, go to “Settings” in the upper right corner next to “Logout”. Select “Privacy Settings”, then “News Feed and Wall” and click the tab “Facebook Ads”. In the drop down box, make sure you select “No one”.
Hopefully that does it – but then again, we can never really be sure what they do with the content we upload, can we?