The problem was finally solved:

1. There was a missing pair of (round) brackets to identify the Name of the image as a string (line 88)
2. The other relevant mistake was on line 60: I expected that the scaling of the CTM scales the number of pixels to a PDF-width. Instead, the "scaling" (parameters "a" and "d") actually provide the width and height for themselves.

After changing

• /Name ATH_TEST-PNG-01-241216a.png (line 88) to /Name (ATH_TEST-PNG-01-241216a.p) (for deomonstration purposes I just stripped the "n" from "png" to keep the xref-positions untouched) and
• 1.000000 0.000000 -0.000000 1.000000 200.000000 300.000000 cm (line 60) to 100.0000 0.000000 -0.000000 100.0000 200.000000 300.000000 cm (this coincides with KJs final statement in his answer.

As consequence the result was as expected: The PDF-viewers show the red test-image inside (more precisely: "above" the outlining blac rectangle (of linewidth 1pt)) ). The problem was solved by KenS initial comment: Many thanks to all comments and answers!