I am also having this issue on Survey123 Connect 3.13.249.
I created report templates with signatures (images) about 3-6 months ago, and everything outputted fine. When I go in now (within the last couple of weeks), I'm getting this error (below) in regard to the signature not being able to export as an image. It only works as text now, which isn't of use to me.
@ZacharySutherby ?