different from Ruby, no C api for exporting image.
so How get scene images without client-side by C api?
The standalone C API cannot do this. The images are generated by the rendered scene and only the client-side application has a renderer.
Thank you for your reply. without client app, How can I export images? Is it ok to transfer skp model to others , like bim, rvt, ifc … and then get images by bim model?
Again, it can not be done with the SketchUp C API alone.
Yes. Your application can read the model data and convert it to another file format.
But the SketchUp C API does not have any output conversion functions. Your application must do the conversion. Look at the examples in the “sample” directory of the SDK package.
you r right. 
so we need add what? any example: a handwritten renderer! 
No rendering example comes with the SketchUp C API.
You will need to search the web for an open source example of rendering a scene in C or C++.
The SketchUp C API is mainly meant for .skp file conversion within other applications.