It's a shame Congress chose a name for this law that yields the misleading acronym ESIGN. The only part of the ESIGN act that deals with electronic signatures boils down to "a signature may not be denied legal effect...solely because it is in electronic form." THAT'S IT. The act prohibits regulations and interpretations and doesn't offer a clue as to what constitutes an electronic signature and what does not.
Most of ESIGN deals with the delivery and use of electronic documents. These are the parts of the law that trigger pre-consent disclosures and the ESIGN test drive. As I read the original post, document delivery is not part of the question.
Although there may not be a laundry list of do's and dont's, associated with the act of obtaining a signature on a loan document, that doesn't eliminate other risks--primarily the risk of repudiation (borrower claims the "signature" is a forgery and refuses to honor the content of the "forged" document.) If you haven't already discussed this possibility with bank counsel, do it now. Take whatever steps necessary to guarantee that a judge somewhere out there in the future will accept that digitized image (which could possibly include an "X" by an illiterate person) in exactly the same way s/he accepts wet signatures and proceeds to grant judgements in your favor.