The exported PNG can have the 96 DPI metadata changed using GIMP or ImageMagick. Here is how to do it on the command line with ImageMagick:
```
-convert -units PixelsPerInch jalview_dmg_background_blur.png -density 72 jalview_dmg_background.png
+convert -units PixelsPerInch jalview_dmg_background_blur.png -density 72 jalview_dmg_background_blur_72dpi.png
```
The final image should be saved as `jalview_dmg_background.png` (which the above `convert` command will do) as this is the filename set in the `install4j8_template.install4j` file to include in the final DMG.