Here's another fun resource of free images for you to scoop up and use: the Dictionnaire infernal, a 19th-century compendium of all things devilish.
You'll have to scan through the book to find all the creepy illustrations, but they are plentiful. Like all the resources at archive.org, the book is downloadable and free. As the Library of Congress (the uploader) says, "The Library of Congress is unaware of any copyright restrictions for this item."
Either on the web or in your downloaded copy, you can then screen capture the illustrations you want to use.
I'll just leave you with some samples.
What's not to like? Halloween will be here sooner than you think.