That's not even remotely comparable is it, one is someone's words against the other(s) and the other is black & white, he either had indecent images on his phone or he didn't.
It goes one or two ways, there's no in between. If you are sent the images you either delete them and go and slap the c**t that sent them you or you report the c**t straight away and keep the messages as evidence and let the authorities deal with them.