Opening a PNG file with VuePrint is a crapshoot. And it is not dependant on the PNG image or path length!
I was fortunate to discuss the iussue with Ed Hamrick. He states that VuePrint does support PNG files and could think of no reason that PNG would cause a problem.
Yet, in my experience, PNG files sometimes open but most often hang the app. It is not a problem with path length or newly revised PNG format. I am certain of this, because if you try to open the same file 10 times (e.g. by double-clicking on a thumbnail it in Explorer), it will open about 15% of the time (one or two successful launches in 10 attempts). Same hit ratio for using space bar in a folder with JPG images -- and then encountering a PNG image.
I currently use Windows 10. I think that the problem also existed with Win 7/8. But I am pretty certain that this was not a problem in the days of XP, ME and 95/98. So if it is critical that VuePrint work flawlessly (without re-launching each time that it encounters PNG), then you might wish to try launching it in Win-98 compatibility mode.
For me, the solution is to simply associate the .PNG extension with an alternate photo viewer (or a heavy duty photo editor).
