|I'm not sure how you could 'overwrite' the pictures. They can be deleted, but how you could 'overwrite' them is beyond me.|
But anyway, you can try a file recovery program to see if they can be recovered. But to do this you should physically remove the hard drive from the system, install it as a slave or external drive on another system to try the recovery.
Here are some options for file recovery
Free but only good for accidental deletes
Zero assumption Recovery
The demo is limited
It will only recover'up to' four folders per run
But you can make multiple runs
Ontrack's 'Easy Recovery'