Effectively, you want to disable Overlay rendering, since you can only NOT take snapshots of video when it renders in Overlay mode, which is usually the default mode as it's the fastest. I don't know if that player has that particular option, but disabling hardware acceleration temporarily and reloading the video will have the same effect.
There are alternate media players you can use, like
Media Player Classic here which will allow you to use something other than Overlay (you can set it in Options > Playback > Output here, VMR9 Windowed if you have DirectX9 installed, or the one of the two Renderless modes will work nicely for screenshots here). This media player does have a saving images function too, but again, you have to be not using Overlay, just the same as for normal screenshots.
Or get
VirtualDubMod, a simple video editor, and use the copy functions in the edit menu, which will copy specific frames to clipboard such that you don't need to do any cropping afterwards to remove the media player's window or whatnot.