This functionality is now complete and will be part of the next version. I plan to submit it to the app store today.
The metadata view will now display an "Asset" tab if there are images or pdf files in the same folder as the selected audio file. This tab will show thumbnails of all supporting files and open the pdf viewer or full screen image depending on type when selected.
On the iPad version, there will also appear an icon on the "Now Playing" screen if the current playing file has additional files for quick access.
Remember that all data files like pdf`s need to be imported into JRiver before you see them.
When you configure auto-import, this option is deselected by default, so make sure you check it.
Same for image files, make sure they are actually imported. Images on OSX are not yet supported by JRiver.