I get why the feelings sucks. You're in your head and somehow only the worst possible outcomes ruminate. You've called the cops, good on you.
You're not in charge of the world, there ought to be others who are charged with taking care of that girl. Their failure in preventing her from showing up on your porch is not your fault either and she's their responsibility, not yours. Keep repeating that in your head.
What do you want to do with your poster? Will this be used as a meme or will it actually go to a printer or be displayed on a high-res screen? If it's just a meme thing, any image creator AI will do. Most of them have free options.
If this will actually go to a printer, you'll need high resolution images and would do well to design it in a vector format, SVG for open source software: Inkscape is free to install on computers, vector-ink is a thing for mobile and the web as well. Adobe Illustrator for the corporate expensive route. Big file sizes work better on a good computer so YMMV.