Generating the Preview of a New Object

You can set the preview picture of your new Object that is displayed in its tool settings dialog box, as well as next to library objects listed in a browsing window. Follow the below steps to do so:

1- Place the object and open the 3D view of the object.
2- Copy the image by drawing the marquee in 3D around the object and go to Edit < copy, you can also copy a photo render of the object or a real life photograph of the object from the clipboard
3- After copying the preview, select the object again and go to file < Libraries and objects < open object.
4- In the Master window of the opened object click on preview picture, go to file < paste and save the changes.