As an addition/alternative to feature request #19688, implement "Rotate 90° Left" / "Rotate 90° Right" options in the image context menu.
For sake of clickability, that's bug 19688. To clarify, are you're suggesting that these options be present when viewing a standalone image, or also when images are embedded in an HTML page?
These could also be present in embedded images, since the rotation won't happen until the user specifically decides to do so by clicking the menu item.