I think there's more to that. A proper digital signature requires you to obtain some certificate/key from an authority which you can then use to sign documents (this doesn't even require an image of your physical signature in the document). This proves that it was actually you who signed the document. The document also can't be altered afterwards without rendering the signature invalid etc.

Just adding the image of your signature to a PDF is probably fine for unimportant things, but it certainly isn't enough to be legally binding (at least in the EU).