In broad sense img to skp conversion describes the use of any 'images' in SketchUp, typically as textures or other objects that can be used in the tool, but you have to be specific for the file type.
As for specific .img file types that can be used in SketchUp, we do not know about any in particular that can be used in the software, so perhaps you have to first convert your .img files to a format suitable for SketchUp.
Updated: November 1, 2023