This is an outstanding effect with a great deal of shock value. Imagine this: the visitors of a haunted house walk down a long hallway with dim light with framed pictures hanging on the wall. As one of your visitors admire a portrait, the picture in the frame slides down exposing a hole in the wall and a ghoul scaring them all.
It is rather simple to create this effect. You have to get prints of various paintings, the older and scarier, the better. Mount them to the walls on each sides of your hallway. You also have to build wooden frames around the pictures, mounting them to the wall to appear as hanging pictures.
Here are the steps to build the sliding picture. Attach the picture to a piece of plywood and then build a track on the back side of the wall where the picture can slide in. A latch on the moving plywood will hold the picture in place behind the fake picture frame. Make sure the latch is easy to work with and has a rapid release. This is vital to maintain the element of surprise. You can build several pictures in your hallway in this way. Thus, if your haunted house is visited for more than one time by the same person, he or she will not know which picture is modified. Keep in mind to make all the pictures look alike, regardless of which are sliding and which not.
For the ghoul to know when someone is approaching it is recommended to drill a small hole in the picture or near the picture. The spot behind the wall should be dark for your visitors not to see any light shining through. To maximize the effect, lighting and special sound effects can be initiated once the effect is set off. The effect might be operated by a handle mounted on the back side of the moving plywood piece. You might also attach a piece of plywood at floor level to prevent the picture accidentally fall on the feet of the ghoul.