Have been able to resolve this at least for Emacs25.3 on Windows 10.
In short, libpng dll's and zlib1.dll's need to be added to the bin subdirectory of the emacs folder. The version number and name of the libpng dll's needed can be obtained by evaluating
(cdr (assq 'png dynamic-library-alist))
These can be obtained at https://mirror.freedif.org/GNU/emacs/windows/. For me the zip file needed was emacs-25-x86_64-deps.zip.
Choose the appropriate file depending on your system, unzip it, and copy/paste the dll's mentioned above to the bin sub directory as mentioned before.