(Photo courtesy: Coconino Sheriff's Office)

An Arizona couple has been arrested for allegedly posting photos of their two young children bound by duct tape and, in one photo, hanging one of the children upside down from exercise equipment.

Kayla Almunia, 19, and Frankie Almunia, 20, told police that the images were meant as a joke, but authorities aren't laughing.

"I don't know how anyone can rationalize or believe that this behavior is in any way funny or a joke," Coconino County Sheriff's Commander Rex Gilliland told ABCNews.com. "Law enforcement believes that this is an act of child abuse."

In the photos, the 2-year-old and 10-month-old are allegedly bound with duct tape around their wrists and ankles. Their mouths were also closed shut with duct tape. Police said one of the children was hung upside down on an exercise apparatus.

Police say Kayla Almunia allegedly posted the photos on Facebook on Wednesday night. Soon after, an anonymous caller reported the images to the police and provided authorities with the couple's names and address.

Gilliland said police obtained 10 photos from Kayla Almunia's cell phone of the bound children. All of the photos appeared to have been taken at the same time.

While police said there were no indications that the children had possibly been abused in other ways, the bondage appeared to be traumatic.

"In one photo, the child did appear to be extremely frightened and fearful and in a later one appeared to be crying," Gilliland said.

The children were initially taken into custody by Child Protective Services but have since been released to what police deemed to be "suitable family members," Gilliland said.

Gilliland said that the anonymous tipster is the person to thank in this case and encourages others to be similarly vigilant.

"We encourage people to pay attention and if they see something that's odd or criminal in nature, particularly on social media, inform law enforcement," Gilliland said.

Each parent was charged with two felony counts of child abuse. They are being held on bond in the Coconino County Detention Facility.

Earlier this month, police in Chicago were investigating a father in a very similar case. Andre Curry, 21, allegedly posted a photo on Facebook of his little girl's arms and legs bound together by thick tape and her mouth covered.

The image was posted with the caption, "This is wut happens wen my baby hits me back ; )"

And in October 2010, photos of a teen mother and her boyfriend duct-taping her 2-year-old son to a wall were made public by Nebraska prosecutors, weeks after they were convicted for the incident in which they took pictures of the toddler for fun.

Jayla Hamm, who was 17 at the time of the abuse, received 10 days in jail and two years probation. Her boyfriend, Corde Honea, 19, an ex-convict, was sentenced to three to five years for child abuse and received an additional 12 to 24 months on felony possession of stolen firearms.