It's by far a stupid question.
This very old page, near the very bottom, says this isn't possible in WinBoard.
This 3 1/2 year-old post says you need the "development" version to do that.
To my knowledge, this feature was never implemented in the main version. I have a copy of the lastest version and I don't believe it's possible.
I recommend Scid vs PC if you wish to do this. With Scid vs PC, you just click the "+V" icon in the VCR button menue, to start a new variation. It couldn't be easier. Years ago WinBoard was my favorite interface, but a few years ago that changed, and I use Scid vs PC for almost all of my GUI needs now.
(Note: Obviously, you mean a PGN file, not a PNG file. A PNG file is a graphic file. 
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I'm fairly new to all this so forgive the stupid question. When I'm analyzing a PNG file, I want to be able to move pieces in the middle of the game to explore a different line of gameplay than is on the PNG, and then come back to the originally-played line. How do I "step out" of the PNG gameplay sequence and explore new moves using this program? I have tried other programs and I've had similar difficulty. There has to be a function I'm not selecting...Thanks!