How would the defense argue to exclude such evidence?Assume that the prosecution in the murder trial of a mother offered as evidence postmortem...

The defense should argue that neither the pictures nor the
video are relevant to the case at hand.  They should argue that both are meant simply to
prejudice the jury against the defendant.

The pictures of
the child are not necessary to prove that the child has been killed.  They are being
shown (the defense should argue) simply to horrify the jury and make it want revenge. 
The film is even less relevant to the case.  The defense should argue that the fact that
a person engages in that sort of sexual conduct has no bearing on the likelihood that
they will abuse their child.  The defense should argue that the prosecution is simply
trying to make the jury feel that there is something "weird" about the mother.  It is
trying to imply that her sexual behavior makes her somehow a bad
person.