Withholding potentially-exonerating evidence from the defense in a criminal prosecution is a big no-no. Of course it happens on a regular basis, but if you've got the money to fight it nicely sets up the basis for a reversal of a conviction on appeal. See: http://www.totalcriminaldefense.com/news/articles/criminal-e....

This is not a search or privacy problem per se. Most situations where exculpating evidence is withheld aren't so clear-cut as photographic evidence of the supposed perpetrator's car in a different place. It's usually something more like a statement by a witness whose exculpatory potential is ambiguous enough that a prosecutor can rationalize it as irrelevant. Indeed, I think technology ameliorates this problem. It's hard for prosecutors to withhold DNA evidence, for example, because juries know its widely used and take it so seriously. If use of photographic evidence in prosecutions becomes common, I think it will be difficult for prosecutors to withhold relevant information from the defense without compromising their ability to get convictions from juries.