Your question initially makes it sound like you're looking for a noun describing the person's job (and then "veterinary pathologist" might be correct, I think). But if you're just trying to translate that sentence in context, I would think that the absolute least ambiguous way to translate it would be:
If they consider it necessary, the person performing the necropsy may take pictures of macroscopic changes reflecting the damage left by the drug.
It doesn't matter what their job description is in general; what matters in this sentence is that they're the person performing the actual necropsy which is being discussed in this sentence.
Alternatively, it's possible that in the original context there is no necropsy necessarily being performed, and that the sentence's oblique reference to "the necropsist" is meant to imply that
If they [who?] consider it necessary, a necropsy may be performed in order to take pictures of macroscopic changes reflecting the damage left by the drug.
or
If they [who?] consider it necessary, the animal may be necropsied in order to take pictures of macroscopic changes reflecting the damage left by the drug.
In which case, the least ambiguous translation might be
If it is desirable to photograph the macroscopic changes reflecting the damage left by the drug, a necropsy may be performed for the purpose of taking such photographs.
At which point we're left with a tautology — the sentence has been stripped to its essence, which is nothing at all. Naturally, "if" you consider it necessary to take photographs, then you "may" choose to take them! We have succeeded in conveying no information to the reader. :)