Short of physically destroying the platters or degaussing the drive, your best bet is to simply make sure that all traces of the picture are deleted, including the thumbnail cache, and then completely filling up your drive with data a couple of times. It's incredibly easy to retrieve files that have been deleted unless they've been overwritten by other data. Really though, due to the density of the platters in modern hard drives, data that's been overwritten is pretty much gone forever. There's a tolerance with the needle that reads data from the platters. It doesn't always hit perfectly on the same spot every time, which is why it's possible to read old data, even if that old data was overwritten. But as drives became more and more dense (larger storage capacities), that tolerance has been lowered to the point that recovering overwritten data is practically impossible, even by the most expensive forensics equipment available.
