Hey Al! I hope all is well with you and Gretchen. We are looking forward to seeing both of you at the Wasatch Fly Tying Expo this coming year. In regards to your question, I hope I can shed some light that you find useful on .PNG files.
PNG was designed back in the mid 90's to replace .GIF files. To give you a bit of history on this reason, 1. The .GIF file extension got patented by a company called Unisys. 2. The .GIF extension has a limitation of only 256 colors. This means that a computer is only capable of displaying 256 colors on a GIF. With technology advancements and computers being more capable of handling more colors arose the need for a different format.
PNG is not to be used or replace professional photos. The reason that PNG has become a file format is due to the transparency options that can be used with it. Most graphic designers/web guys use PNG files to handle any kind of transparencies for logos etc. A GIF file is limited to 8-bit and PNG offers a color depth of 24 and 48-bit (True Color). This results in smoother fades and much more controlled color outputs.
The reason you would not replace a JPEG file with PNG is the fact that JPEG can produce much smaller files than a PNG on photographic images. This is because JPEG uses what people call a "lossy encoding method" that is designed specifically for photo data. Using PNG instead of a high-quality JPEG would result in a enormous file size increase, typically 10x's lager and you usually do not see any kind of improvement using the PNG file.
PNG is a much better choice for images that contain text, line art. This is why you see most graphic designers who do vector art or using Adobe Illustrator, saving files in PNG format. Another reason you would save in PNG is because JPG does not support transparency. If you are going to be saving images that require a lot of editing in the future then you can justify saving the files in PNG format. Otherwise do not use it. In the past if you wanted to save something as a transparency, you would use GIF. GIF is still nice for smaller types of logos and images and cuts down on file size. I almost always use PNGs now!
I hope this information helps you Al. There are also some nice links out there that go over this topic in full detail. Bottom line, I only save to PNG when I am working with images in Adobe Illustrator and doing some sort of logo or design that encompasses a photo or is a file with lots of gradients.