The only contest that has a monthly "theme" is the Photo of the Month...which you can see listed in the winner's box on the home page...as well as a monthly theme update stickied here in the Site Info forum.
BPOM (Ball Python of the Month) and HOTM (Herp of the Month) are both just what they say. Pictures must be taken by the person submitting them, and of animals that you actually own. (You can't submit animals from someone else's collection, shows, zoo pictures or wild herps, etc) These pictures can be funny or cute or hot or whatever you'd like to show off so long as they fit within the rules and the category.
Hope that answered your question!