The selection of PNG-supporting image viewers for VMS (or OpenVMS nowadays) is rather limited; indeed, I am aware of only two viewers, both ports of popular Unix/X viewers:

ImageMagick display

Version 4.2.0, John Cristy. Conversion capabilities (mostly via accompanying convert utility); full gamma support; reported to include chromaticity support; partial MNG support. There is also a 32-bit Windows port, but it requires a third-party X server to run.

http://www.wizards.dupont.com/cristy/ImageMagick.html

XV

Version 3.10a, John Bradley. Conversion capabilities, including interlacing support but without the ability to write transparent PNGs; full gamma support; preserves text information. XV is widely considered to be the preeminent image viewer for the X Window System. The only major drawback is that it was last released in December 1994, five days before the CompuServe/Unisys GIF announcement that began the PNG saga, and therefore does not include PNG support in the default distribution. Fortunately, it is available as C source code, and the home page includes not only the PNG patch but also several others, so it can be recompiled and tweaked at will. An upcoming patch will allow an image-background color to be set, similar to the -bgcolor option in the demo viewers in Chapter 13, "Reading PNG Images" and Chapter 14, "Reading PNG Images Progressively".

http://www.trilon.com/xv/