Well.. I think that it can be 'restored', but it will not be, by far, "the same thing" as the original picture..
I think it is something similar to 'the first rule of equalizing', "what was lost is forever lost.. but there are ways to imitate what was lost" (like.. if you cut all the sound from a certain frequency on the mixer, that specific sound is lost forever, can not be 'raised up again' on the EQ/expander/etc.., but some 'soft' non linear parametric EQ's can, for an example, 'interpolate' between frequencies to imitate the 'lost sound', but it takes a good 'sound engineer' to do that, and the 'recovered sound' is indeed not the same sound that was lost...) .. So.. i think that if it is an expert photo restorer, it can be done.. but, depending on the case of course, it can be a very hard and tricky job.. The 'lost' information on the picture will be replaced by the restorer's 'artifacts' to imitate what he would suppose to be in the original one..
But, of course, it all depends on the condition and quality of the image to be restored.. it depends, can be an easy one too =D
