That is simple!
Copy your png2eps.exe to system folder(e.g. c:\Windows\ or c:\windows\system32).
After that, create txt file and put in it:
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\.png\shell\png2eps]
@="Convert PNG to EPS now!"
[HKEY_CLASSES_ROOT\.png\shell\png2eps\command]
@="png2eps CMD \"%1\""
Where: CMD - cmd switches and options for png2esp. \"%1\" - input file name from shell.
save txt, and change it extension to reg(e.g. before: file.txt after: file.reg). 2click on reg file and import to registry.
Menu item "Convert PNG to EPS now!" you will be see in context menu only if you call it for selected png file(s).