The Web Design Library site is a fantastic resource for Photoshop tutorials and ideas, and their “special effects” tutorials are a great source of inspiration.

I was recently digging around on the site and a tutorial for adding a ghost to a photograph caught my eye. I grabbed a couple of family photos I had handy and with very little effort was able to combine this and this to create this (ghost dog! Aiyeee!) I particularly like that the tutorial includes instructions for desaturating and adding graininess to the photo to make it look like the sort of low-quality picture that “real” ghost photographs always seem to be.*

A bit more searching turned up instructions for several other spooky effects, so if you have access to Photoshop or a similar photo-editing tool (here are some free alternatives) you might want to give some of these a try:

Ghostly Effects – Add a spooky silhouette to a hallway.

Night Vision – Make a photo look as though it were taken with a night vision camera. This would be useful for creating something like the Louisiana Swamp Monster photo that was making the rounds a few months ago.

Adding Realistic Fangs – Nice way to add subtle vampire or werewolf fangs to a portrait.

Zombifying a Photo – This really looks more like plain blood spatter than decay, but it’s still a neat effect.

A couple of more complex multi-photo manipulations:
Forest Magical Scene
Night of Spells

Create a Classic Horror Movie Poster – I especially like this one because it would make a great wedding invitation for a couple of horror buffs. You’d probably want to play with the basic idea to include photos of both partners and perhaps give it some sort of wedding-themed “film title,” but it would certainly be a unique presentation.

*A note if you try this one yourself: Steps 3-5 of the instructions involve adding a layer mask and applying the background image to it. I have no idea why, because it doesn’t seem to add anything to the finished image. I didn’t bother.