At least two websites provide free witch clip-art drawings without watermarks. Commons.Wikimedia.org offers free witch art and photos in several categories, including in paintings, statues, icons and costumes.
Other categories at Commons.Wikimedia.org include witch hunts, trials, sabbaths, witches on brooms and witches in film and heraldry. Most of the images at Wikimedia Commons are copyright-free, but some are suitable for adults only.
OpenClipArt.org offers two pages of images with more than 40 witch drawings that can be downloaded without watermarks for free.
Clker.com also gives dozens of free witch images without watermarks, though some of the drawings are the same as at OpenClipArt.org.