I’m confused on the question - what are you looking for?
If you are looking for a way to digitally make graffiti like images, you’ll probably need to use a painting application. I know that it is possible to make some cool art with Krita and Gimp, both open source image manipulation programs, but I’m not sure how well they support graffiti-like art.
If you are looking to scan graffiti and turn white into transparent, then you can probably use Gimp with either the lasso select or color select tools, select the white areas, add an alpha channel to the image, and then erase all the white sections. There should be tutorials on how to achieve these steps if you look for them in a search engine. (For example “gimp select all color” and “gimp add alpha channel”)
For precise drawing, you may want to potentially look at drawling tablets for PC (Windows, Mac, or Linux depending on what you are using) and try that. Then you could get good precision using the drawing tablet similar to using a pencil and paper.