1. Here's how

The thumbnail images for Imported Models are updated automatically when you import and re-import the models.

1.1: To create your own thumbnail image for an Imported Model, place the model in the Scene.

1.2: Select the model and make sure that the thumbnail image of the model is still selected in the Imported Model Library.

1.3: Point the camera at the model.

1.4: Hold down the CTRL key and left-click the thumbnail image in the lower left corner of the screen in Build Mode to update the image.

Before:

After: