Editing Texture Maps of Polygons

Ok so when I first get in and import my model I go to one of the polygons on the model and I right click into "Open Material Editor" and I go to "Texture Maps" to change the texture. My problem here is that it will not let me change the texture at all, it is grayed out and won't let me touch it.

Any Idea on how to fix this so I can change the texture?

Start by reproducing the issue with an imported model: select a polygon, open Material Editor, and inspect Texture Maps. Trace why the texture control is disabled and identify the relevant material-editor entry point. Done means the texture can be selected or changed for the affected polygon without breaking other material editing.

Written by the indexing model from the issue text.