If the image is free to use, it's good to put a coryright below, like 'Image: John Johnson, redsnickers.com'.
The other story, if the image is not free. Then you can't use it. That's why make an image google search for each and see whether it isn't listed in any stock bank.
Adsense doesn't deal with copyright directly, but if there comes a time for manual assessment, the stolen images without copyright would definitely be a negative factor for your blog. The borrowed images with corypight - more likely positive factor.
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