Q: I ran a couple of small blogs and unknowingly used copyrighted pictures.

A: Should you worry?

If no copyright holders have contacted you regarding infringement, you should not worry.

Do I have any suggestions?

You "nuked" the site already, which is a great first step. Delete all copyrighted images you have from the server, it's not enough to disallow search engine indexing. For added peace of mind, try to find copies of your website online, search for the URLs and see if anything comes up, search on archive.org. A copyright holder would have to find evidence of infringement before there would be a reason to sue you.

You are correct that merely attributing credit on the images is no enough to prevent you from getting sued. If the images have not been expressly released into the public domain by the author of the work (photographer), then you must obtain permission and usually pay a licensing fee to use their images online. Every image is copyrighted with the click of every camera shutter. However, if a copyright holder registers their work with the U.S. Copyright Office, they can seek statutory damages of up to $150,000 per infringement. Generally, however, a single image infringement case will settle privately or in the courts for about $2,000-$5,000. Each case is decided on its own merits, that range has no bearing on what might happen if you were sued for copyright infringement.

If the images were copyrighted, a copyright holder can still sue you for copyright infringement if they can prove that you used their images without their permission. Website like archive.org provide snapshots of websites in the past, you could search there to see if there is evidence of your blogs using the images you're concerned about.