The issue with timestamps Chris was mentioning is already tracked in another bug: #792202 and also sent upstream [1]. And upstream was already discussing about making the Timestamps and the ID turned off by default (and only embedded on request) [2].
This bug is actually only about the behavior with the example attached by Sandro. Ignoring the Timestamps and the ID field, there is some additional unreproducibility with those specific PNG files (perhaps related to converting them or so), which should be fixed. [1]: https://www.tug.org/pipermail/pdftex/2015-July/008952.html [2]: https://www.tug.org/pipermail/pdftex/2015-July/008955.html
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