In the Content Manager and Content Libraries dialog boxes, AutoCAD Mechanical toolset displays a thumbnail for each level of the tree.
When you create a new part/feature or view, AutoCAD Mechanical toolset automatically creates a thumbnail of the geometries. AutoCAD Mechanical toolset updates the image every time you save the image.
If required, you can change the thumbnails, by right-clicking the thumbnail and selecting Change Thumbnail. You can select a JPEG file, or a BMP file. When you assign a custom thumbnail, the image stops updating automatically.