add object to domainmodel associated to person and contract, inheriting from image. place the png in that object and connect to the correct person and contract
Include the image in the printed version
remember to include a disclaimer on the use of the signature in the mobile part of the app, and a tick to have the signer declare he knows this signature is used for that one purpose only
when placing the signature on the contract, try and position it to overlay a fixed text part to deter "sig-sniping"