Additionally, they claim that Arias gave two different accounts of the incident, telling one expert that the images were on Alexander's computer, and later testifying at a hearing that that they were hard copies of photos spread out on his bed.
But Stephens ruled on Thursday that she would allow the testimony today.
"I'm surprised the judge let this in," said ABC News legal analyst Dan Abrams. "This is incredibly inflammatory and it fits conveniently into the defense's theory."
"This is a risky move for Jodi Arias because if these jurors don't believe her already, this is just going to make them angry at her," he added.
Arias' defense team has also tried to include 10 letters that they say Alexander wrote as evidence of his attraction to boys.
Handwriting experts for the defense claim that the letters were written by Alexander, while experts for the prosecution say they were forged.
The letters may be admitted into testimony later this week.
The defense may also play an audio recording of a phone conversation between Arias and Alexander from 2008 that is sexual in nature.