To temporarily disable synchronous changes to images,
use the icon 
in the upper right corner above the image, similar to synchronizing series in CT/MRI
To the right of the icon is a down arrow button 
, allowing you to set synchronization parameters
in particular - preserving the black background during the positive/negative
The 
Reset tool resets the settings of the active image and enables default synchronization
If the image was synchronized with others, their settings will also be reset to original values
Synchronization between two mammographic studies is not available
To have synchronization disabled by default, uncheck the Synchronize W/L, scale, shift option in the Configuring the Mammography Specialty window