Hello,
Thank you for reaching out to us. @Dunhou is right, the Picture Viewer is sealed, which means that you can put images into it (with c4d.bitmaps.ShowBitmap or c4d.documents.LoadFile) but you cannot get images or information out of it. This generally applies to all UIs, we do not expose UIs, but the data structures behind them. So, there is for example no Object or Material Manager API interface, their functionalities are exposed via the scene graph. For the Picture Viewer there is no data structure exposed.
Cheers,
Ferdinand