Need advice on setting License, Copyright on PNG images

[CricketX]

I'm going to submit a bunch of PNG images to a contest.  I'd like to set the Copyright, License, and comment in each of the images properly so that all OS's and software that most human populations use will clearly show I own the image and where exactly they stand on how they can use or modify it.  I do not want to put a watermark on the images as it will be distracting, I think.

I guess I just need to know, from you photo people who have more experience that I, if the code below is enough, and if not what else should I do:

exiftool -Copyright="CricketX (c) 2021" 001.png
exiftool -License="https://creativecommons.org/licenses/by-nc-sa/4.0/" 001.png

One of the biggest reasons I'm asking this question here is that the 'License' tag does not seem to be in the published exif standard.  I really want the Copyright and License tags to be there when people are looking at the metadata in whatever OS and software they use to view metadata on images.

Any thoughts or experience to share about this?

[Phil Harvey]

I would look to the MWG recommendation for where to store the Copyright.  See the MWG tags here.

The License is probably best written to XMP-cc, which is what you are already doing.  You're right that it isn't part of EXIF.

- Phil

[StarGeek]

I'm going to submit a bunch of PNG images to a contest.  I'd like to set the Copyright, License, and comment in each of the images properly so that all OS's and software that most human populations use will clearly show I own the image and where exactly they stand on how they can use or modify it.

You can embed the data, but it's not going to be visible to someone who doesn't know how to look for it.  The problem is that there are very few apps that will actually read metadata in PNG images.  For example, under Windows Properties, the only thing that will be displayed is the "Date Taken" property and that will be displayed only if the relatively obscure PNG:CreationTime tag is set.  Windows won't  read any other metadata in the file.

I would suggest converting the PNGs to Tiffs.  It would be a lossless transformation as long as you don't choose jpeg compression (choose either LZW or Zip compression).  Tiff files are well supported in most OSs and programs.

The MWG tags are the best place to put any information you want to be more visible.  Those are commonly supported.  The Creative Commons License tag, while technically a good place to put it, is probably never going to be seen, as I have yet to come across a program that will actually read the Creative Commons tags.

For licensing data, I would suggest the XMP:UsageTerms tag.  Maybe the XMP:Instructions/IPTC:SpecialInstructions as well for completeness.  These are part of the IPTC photo metadata standard. They are not readily visible by OSs, but will be visible in programs such as LightRoom.  The definition of the XMP:UsageTerms tag is
     The licensing parameters of the image expressed in free-text.
     Enter instructions on how this image can legally be used
while the instruction tags say
     Enter information about embargoes, or other restrictions not covered by the Rights Usage field