A step-by-step guide for creating Boulders in SketchUp using Artisan Extension
https://www.sketchup4architect.com/boulders-in-sketchup-using-artisan-extension.html
The first step is to enable Artisan Toolbar. After that, you should go into settings and allow the creation of a proxy from the group. Make a group by creating a box and selecting all items in it. If you click Subdivide & Smooth, you can smooth and divide your image. Double-click the boulder after you do that. Then you click the extrude button.