Re: I'm not sure you can make the its not porn argument
"Plus a private person (the computer tech in this case) is not bound by the warrant requirements that a law enforcement agent is."
While a private person is not held to the same the requirements of a law enforcement office, it is still the -prosecutor's- burden to prove that evidence used in obtaining a search warrant is valid and can be upheld in court as required by the 4th Amendment. The defense is merely arguing that the prosecution did not do their due diligence in demonstrating that the accused has indeed committed the crime of possessing child pornography. Given that the "image was pulled from unallocated space on Rettenmaier's hard drive" does not in anyway show that the defendant intended to -possess- illicit material (even if was legally considered illicit material in, and of, itself). In fact, the fact that it was deleted demonstrates that the accused did not intend to possess.
Of course, it could also be argued that since the image was deleted (By virtue of being in unallocated space of the hard drive) that, depending on the repair requested, the technician had no cause to even incidentally discover the image and was rather attempting to recover all images on the system to be used for purposes beyond the work contract signed between the accused and Best Buy.
A garbage can submit evidence to the police and is usable so long as the evidence was discovered during reasonable performance of their work. In you example, yes the garbage man could turn in a bloody piece of clothing to the police to be used for a warrant, but it would only be admissible if it, say, fell out of a garbage bag, or was clearly visible in the garbage can. If the piece of clothing was in a sealed bag and no evidence of a crime was visible without opening the trash bag, the piece of clothing would become inadmissible and unusable for gaining a warrant. Even then, the evidence is still useful to the police if they only use it as an indicator where the crime may have taken place and could then canvas the neighborhood and if the neighbors report suspicious behavior by the suspect or something like the sound of a woman screaming coming from the suspect's home, then the police can obtain a search warrant that would be upheld in a court of law.