Hard drive read/write heads are incapable of actually resetting the sectors of a hard drive back to a null state after deleting or formatting the drive. This is how you were able to recover the photos. Just about any software that can read the sectors should be able to recover it.
On a side note, those "secure delete" programs don't actually help much. While they do overwrite each sector with a random pattern to help keep files from being recovered, they don't always work. The DOD used to allow these data wipe programs for removal of sensitive data, but a GAO audit found that they were able to recover information on hard drives after those programs were run using simple software tools. Had they taken it a step further and used an MRI to image the drive they could have recovered more. They also found that there was a disturbing amount of leakage of secret and top secret data to systems that were not classified to handle such data due to networked systems. So the current mandate from the DOD is to destroy all magnetic storage media for systems that store sensitive or higher data.
Remember the Alamo, and God Bless Texas...