convert -size 19992x16500 -depth 16 gray:FRAME_1005_H1.IMG out.png

The dimensions are specified in the .LBL files in the same directory, right at the end of the file.

To understand the .img files format refer to the corresponding .lbl file e.g.

    OBJECT         = IMAGE
      LINES        = 16500
      LINE_SAMPLES = 19992
      SAMPLE_TYPE  = LSB_UNSIGNED_INTEGER
      SAMPLE_BITS  = 16
    END_OBJECT     = IMAGE

Instead, the same in cca 100 KB:

https://pds-imaging.jpl.nasa.gov/data/lo/LO_1001/EXTRAS/THUM...

To get some idea of how detailed it is, here's the crop of the 6 MB PNG version:

https://ibb.co/jjoe7y

Take a look at the top bar. The actual resolution (one small piece cropped from the full resolution) is:

https://ibb.co/hthLfJ

That's how astoundingly amazing the actual resolution is. You can see the grain of the film and the sharpness of the reference patterns. Note, that's the technology used in the spy satellites even more than 50 years ago, all without microprocessors which didn't exist!

Comparing with that, the actual shot of the moon surface is, at least in this sample, much less detailed than what the film was able to capture, the scanner to scan, the transmission circuits to transmit and the tape to store:

https://ibb.co/nLhsud

The black dot is maybe a piece of cosmic rays passing through the film.

And if you are just interested in the general look of the moon surface, try first:

https://www.google.com/moon/