I may be over concerned, but after hearing so many banned publishers, I'd rather ask this question now than later, is there any concerns about "free" images that found on the net to be used on my web site for Adsense? What I mean "free" is that there is no watermark on the image, there is no copyright mentioned (not even Creative Commons), and there is no way to tell who is the original arthur of the image. I'm not talking about selling pictures, for example a blog post that I may insert some images to "enchance" the post.
Google does not have any problem you having images on your site. But, they have problem with how you place images. You should not place images next to pictures. Google does not recommend using pictures to enhance your CTR.
A person can't actually use images freely without the consent of the owner and it would be better and ethical for you to seek his/her approval prior to using it. Contact the owner and offer a free backlink to state where the images come from. I'm sure the owner will gladly accept the condition.
Thanks for the reply, I know the ads shouldn't be placed near images, my concern is more relate to copyright and Adsense TOS.
Most of those images are not in the public domain, and you do risk breaking the Adsense TOS for the use of copyrighted images without permission. There are also stock photo companies, such as Getty Images, that search the web for people using their content without paying. If they discover such an image on your site, they could not only complain to Adsense and Google, but also end up requiring you to pay them thousands of dollars.