You can start from sample code in the GNSDK package. Look for /wrappers/gnsdk_cplusplus/samples/sample musicid_image_fetch, and search for function "fetch_image".

I tried that but only returns an image from a local database or something like that and I want to return it for the specific album that I have identified with musicid stream. Somehow I need to get the GDO of the specific allbum but I don't know how to do that.