I went out and was able to capture interesting abstract textures that are creepy/spooky-looking. These make a perfect addition to photos that have a dark/scary tone, to add that little bit of extra abstract texture element.
You'll find here a pack with 37 high-resolution creepy overlays that are absolutely free to use. Upon request, I can create more too if this pack ends up being popular.
To add one of those overlays to your photo in Photoshop, just add it as a new layer on top of your image and then turn the blend mode of the layer to Screen. You can then tweak the opacity of that layer so that you get something that works for your image, and also don't hesitate to erase parts of the overlays where it doesn't compliment the photo. To learn more about how to use textures and overlays in Photoshop, you can refer to this post.
Free Creepy/Spooky Overlays
Below you can see thumbnails for the 37 creepy overlays. Just click on any of the thumbnails to download a full high-resolution version of the texture.