PNG or JPG are not electronic signatures.
Wrong. I've done this before, and just did it again (on a different computer with a different Reader install). Also:
Note the support for JPG, PNG, TIFF etc.
Anyway, I did it on my other computer like I have in the past. I'll have to check my version of Reader that is installed and find out why that install doesn't allow it.
There are two kinds of electronic signatures: e-signatures, which are just stamps (and that includes images like PNG or JPEG), and digital signatures backed by PKI certificates.
Has the first first signature been a digital signature? If it were then you cannot apply an e-signature after that because it will break the previous digital signature. Once a digital signature is applied to a PDF the subsequent signatures can be only digital signatures.