First, nice job and good luck on this project
Personally, i do not like modifying someone's else graphics and for this reason i scan regularly for CC0 or CC-BY licenses for my graphics, even if those graphics are sixty (60) years old like the Dracula (or even older). In your case, things are a lot easier since you just need portraits and small things in general, not whole scenes.
You can find lots of dracula photos in "Google images" and filter them by "Labeled for reuse with modificatation" (allowing commercial use as well) and then you are free to do whatever with them without problem.
Another source for CC0 (free) images is "
https://pixabay.com". Another one is wiki's public domain imagery at
https://en.wikipedia.org/wiki/Wikipedia:Public_domain_image_resources
Finally, you can find LOTS of medieval portraits on Wiki's public domain directory, which you can use freely since the author has long died (more than 100 years old dead, simply called PD-OLD-100 and also labeled as "public domain")
One example is the following link of Shakespeare
https://commons.wikimedia.org/wiki/File:Shakespeare.jpg