He can most likely take the girls for a trip, but he can't just "not come back." That is considered parental kidnapping. If you have caught him taking pictures of the house, most likely he is planning on portraying you as an unfit parent to try and get custody. You say that you are in the middle of a divorce. I assume this means that you have a lawyer. If not, you need to get one immediately.
1) Tell the lawyer what he is doing
2) Of course do what the lawyer tells you to do, but I would file for a legal separation. This will figure out a custody schedule, and give you a legal right to live in the house, and change the locks if you wish.
3) If there is nothing legal right now about you guys not living together, he can come and go out of that house all that he wants to. He also has no legal obligation to pay for a darn thing (although this will look really bad in court).
The ultimate advice that I can give you is to tell your lawyer everything, and have the lawyer petition the judge to stop this nonsense. At the very least, telling the judge what is going on could potentially head off anything else that he might try to bring up later. If you don't have one, GET A LAWYER. If you can't afford one, you can go to Legal Aid and they will represent you for free.
Also, stop doing all of the things that you have been doing for him. It won't help you in the end, and you can see that he is not reciprocating your generosity.
And, if you wanna be a little devious yourself...pretend you have to look at his computer one more time and delete all of those pictures. Then, he doesn't have any "proof" against you.