Currently the JRiver Sidecar file format works only with video files. Support for other formats has been requested (images, documents, untaggable audio formats), but I don't know if it is on JRiver's to-do list.
For now, you can import external metadata by using "mpl" xml files. You can study the mpl file structure by creating a playlist of a few tagged image files and exporting the playlist in mpl format.