I don't think it is just that. In the dry cleaning example, the evidence would be extremely slight, it would turn into a he-said/she-said case, and either no one would get convicted or there would be enough evidence to convict the dry cleaner. And the dry cleaner can do this at most once before the police became suspicious. The cops will probably be suspicious right away: either the drugs are valuable and no one would actually leave them behind, or it is something like weed and the consequences will not be severe enough for the cops to bother.
In the case of the images, the evidence might be just as slight, but the police have essentially communicated to the employees that they won't be looking too deeply at the evidence and juries are ignorant when it come to technology and punitive when it comes to 'illegal images'.
The argument here as the defending attorney is ridiculously obvious: the employee has both motive and incentive; but I'd still give high odds of conviction (or worse, a plea bargain) and even if acquitted the accused will be marked for as long as people remember.