Can't answer the second question.
But for the first, there are many ways to do this.
Right click on the image in Book View or in its image tab, and select:
- Open With to view/edit it in another program.
- Save As to save just the file to disk.
- Copy Image to copy the image to the clipboard.
To replace the file, just click Add File (the + icon), and it will prompt you to overwrite it. Or use Insert Image/File and select it from disk and it will prompt you to replace the same name.