BUG: ISO.get_postcard() format is GIF rather than PNG

Even though the postcard is saved with the `png` extension, it's a gif file.

I run into this as I was trying to get rid of pulling in an extra dependency for the postdoc example in the docs.

Start at the ISO.get_postcard() entry point and reproduce the saved-file format versus its .png extension. Done means the postcard is actually encoded as PNG when saved with that extension, with the existing postdoc example still working.

Written by the indexing model from the issue text.