Tom Sandoval smuggled some contraband into the production of "Special Forces," according to Nick Viall ... but it wasn't snacks or booze -- it allegedly was photos of Rachel Leviss.

The "Bachelor" alum spilled the tea on Thursday's episode of his podcast, "The Viall Files" ... saying Tom, his fellow cast member on the upcoming season of the competition series, brought pics of him and Rachel onto set -- which Nick says was against the rules.

Nick told his guest, Sai De Silva, that there were certain things they couldn't bring during the rigorous training process, adding he would've loved to have brought photos of his fiancée Natalie Joy, but he couldn't.

What's more bizarre, Nick claims Tom was showing the pics of him and Rachel around to the rest of the cast. Nick notes the "Vanderpump Rules" star seemed to really care about Rachel, for what it's worth.