A lot of people take pictures of signed paper logbook entries/endorsements, and attach it to the flight. That's totally fine, but getting precise here, that too is recording the *fact* of the signature, but is not itself the signature: your paper logbook is the actual signed logbook in that scenario.
If something has been signed electronically, having an image of that that is also fine, but it's just a representation of the fact of the signature; it is not a signature per se. Please don't confuse them.
To Mark's point above, it's not just the trail that a digital signature offers. A digital signature offers non-refutability (you had to authenticate yourself in order to issue the signature, so if it's there then you can't deny that it was you) - the bar is greatly raised on forgery because someone has to hack into your account. It also offers non-modifiability: if the entry is modified, the signature is invalidated. (Whereas on paper, you could always change a "1' to a "7", or add some instrument approaches that were otherwise blank...) A few other things as well. See
https://myflightbook.com/logbook/Public/CFISigs.aspx for gory details of requirements (from AC120-78A) as well as MyFlightbook's methods for compliance.