"To my surprise, some of the postcards contained nudity," Rueda explained in a message posted to Facebook. "Some students expressed discomfort about some of the images, so I immediately took back from students the postcards I felt could make students feel uncomfortable."
The teacher encouraged the students to discuss the paintings with their parents, according to Salt Lake City's Fox 13 News. He later learned of some parents contacting the school in protest, as well as a pornography complaint to the county sheriff's office.
"We got some of the pictures and showed them to the County Attorney's Office, and they said these wouldn't meet the definition of pornography," Cache County Sheriff Chad Jensen told the Journal . "They declined to file charges."
Still, Rueda was terminated on the following Friday after declining to resign, he said. He said he planned to appeal the termination.
Rueda and the sheriff's office estimated the set of cards was purchased by the school about three years ago, according to Fox 13.
Follow Josh Hafner on Twitter: @joshhafner