Poor Peter. Without more evidence we can never know how he died - if there was no lid on the tank, it's very possible that he did jump out as said. There's no way of knowing.
I am appalled that someone sent you that photo. As suggested, I urge you to make a complaint to the first boss who isn't involved or HR if the company is big enough - pick someone who's neutral and can do something about it, anyway. Put together a record of everything that's been said and done, as well as everything that's been sent to you, and take it to them.
Keep your cool and stay professional, as you have been doing, but you need to get this stopped. You shouldn't have to put up with this for doing your job.
I am appalled that someone sent you that photo. As suggested, I urge you to make a complaint to the first boss who isn't involved or HR if the company is big enough - pick someone who's neutral and can do something about it, anyway. Put together a record of everything that's been said and done, as well as everything that's been sent to you, and take it to them.
Keep your cool and stay professional, as you have been doing, but you need to get this stopped. You shouldn't have to put up with this for doing your job.