You can add scanned PNG image of the signature to Inkscape, vectorize it, then remove PNG image, save the draw as PDF and use it as image in the LaTeX file. Then the menus to save the image disappear. Using the signature as a layer on top of a PNG image, the option to save the image will available again, but only to save the PNG background, simulating some anti-copy mechanism. A trasparent PNG of top of the vectorized also will work in thi sense.
Moreover, making some small modification of the signature (for instance, a very very tiny text inside some trace, invisible without a huge zoom) the screenshot will not reproduce exactly the signature, so it can be checked easily that is not the original, as it will be pixelated after some zoom. However, it can be extracted unaltered with Inkscape, so there are not secure methods, except use also a digital signature.