You actually have options, alot can be done with text, music and some pics that dont seem "haunted" untill you put them in photo shop.
Another thing to focus on is characters. Get a story for your show, get a few characters and costumes together and have a photo shoot. It could be in a patch of woods, under a bridge, near an industrial setting. Make the pics focus on the characters as opposed to the haunt.
You can also take pictures of victims- alot of mileage can be gotten of an image of a woman lying on the ground (in foreground) and the legs of a killer seen standing behind her.
Take a pic of one of your characters, but take it from inside a normal house with maybe a family watching tv inside, thats a super creepy image and you dont need your haunt to be built yet.
Of course you can buy art from Brainstorm also, and several web site companies have themes that work great.
Im sure others will chime in too!