Imaging and preservation
Frame all four corners, margins and the page number. Position the camera opposite the centre of the sheet. A crop of a signature can show individual strokes, but it leaves out the signature’s position relative to the printed text and nearby markings.
For a signature beside a corrected amount, keep a full-page image and a close-up showing both the amount and the signature. Label the close-up with its page number. Avoid forcing a binding or fold flat merely to make a better photograph.