Welcome, Felix:
Explore the woodworking forum, and for that matter any other subforum for general ideas, and ask questions when you need to.

Blender has this - to make a mesh from a picture "displace-modifier"
Blender is lighter even than SketchUP and this carving modeling is by nature manual work. So, You can start with displace-modifier and tweak it with excellent blender mesh modeling tools which SketchUP lacks.

http://www.davidjarvis.ca/dave/blender/tutorial-16.shtml
http://www.gotmyidea.com/67998-blender-tutorial-displace-modifier.php
http://www.blendercookie.com/2010/04/06/tip-adding-extra-detail-with-a-displace-modifier/
http://forums.sketchucation.com/viewtopic.php?f=323&t=31339&hilit=Rhino&start=15
http://www.blendermasters.com/cms/en/materials/displacement-maps-3d-objects-based-on-2d-maps-3.html

Stick with Blender - if You insist to learn the basics they will became your second nature - I'm a Blender newbie and even made my keyboard shortcuts in SketchUp as in Blender.
http://www.amazon.com/Beginning-Blender-Source-Modeling-Animation/dp/1430231262