@JoergWenzel
Are you saying that JuiceboxBuilder-Pro processes your PNG images when the the 'Resize Images' checkbox is deselected?
This should not happen (and it does not happen in my own installation).
I've just double-checked and, with the 'Resize Images' checkbox deselected, JuiceboxBuilder-Pro v1.5.0 just copies the source PNG files across to the gallery's 'images' folder (but with .jpg file extensions) with no processing or resizing at all.
The files in the gallery's 'images' folder are identical (byte-for-byte) to the source PNG files. (I've checked their SHA256 hashes.)
As long as both the 'Resize Images' and 'Use Watermark' checkboxes are deselected, the source images should be copied across to the gallery's 'images' folder untouched.
Please try again and make sure that you deselect the 'Resize Images' checkbox (like in your screenshot) and save the gallery to a new empty folder (to ensure that pre-existing files are not present in the gallery folder).
If you find that images are still being processed when the 'Resize Images' checkbox is deselected, then I cannot explain why but it might be wise to try uninstalling and then re-instaling JuiceboxBuilder-Pro to see if this makes a difference.