Use this folder to store Photoshop compatible plug-ins to be used with NIH Image. Acquisition plug-ins appear in Images's Acquire sub-menu, export plug-ins appear in the Export sub-menu, and filter plug-ins appear in the PlugIn Filters sub-menu. Image looks for this folder in either the same folder as Image or in the System Folder. 

The "Invert" filter plug-in is a minor variation of the Think C example filter that is part of the Adobe developer's kit in the plug-ins directory on zippy.nimh.nih.gov.