You can use this widget so that people can sign: https://appstore.home.mendix.com/link/app/107984/Mendix/Signature
And you can use images in document templates. And then use those templates to generate a pdf for instance.
You need some code for this. You might want to use https://www.npmjs.com/package/html-to-image It will take a screenshot and inmediately download this.
You can try to generate a PDF and download it as soon as he is generated? It is not a image but it “cannot” be edited.