In reading the reviews for several of these window projectors on Amazon one can get a feel for what can go wrong. The first problem is these window projectors work best if there is less lighting in the room. Buy the ones with the greater illumination power, less illumination tends to wash out in interior background lighting. This is less of a problem for Halloween, since ghostly images might actually add to the effect desire.
One reason for having too dull of an image is improper placement of the projector. The larger the image, the less bright it will be. The projector puts out a certain amount of light. Spreading that light over too large of a surface area makes a duller image.
The size of the image is determined by the distance from the window the projector is placed. If it is too close, the image will not fill the window. If the projector is too far back the image will be too dull, and only a portion of the image will appear in the window. And you do not want to place the projector off to a side, since this will distort the image with one side enlarged more than the other. So, one question one must ask is whether the projector can be placed in a direct line with the window, or will furniture inhibit proper placement.
The projector may come with a tripod, so getting it the proper height is not as serious of a problem as getting it the proper distance back. Some have a base, and are designed to be set on the floor. Certainly, for exterior use the stake is important, so the tripod is not likely to be needed.
Some images are less demanding of exact placement than others. A group of floating ghosts can be larger on one side, or on the bottom of the screen, and being brought into a sharp image may be unnecessary.
Some negative comments involve the tripod legs breaking. Do be careful when assembling the tripod. If it were designed as durable the price would be much higher. This is an occasional use decoration. Of course a damaged tripod can be substituted by having decorative, Halloween based decoration upon which the projector can be set.