As I wrote in the comment above the addon has been completely rewritten. You can try it again and let me know what you think about it. I hope all the problems you've encountered are solved.
N.B.
The Easyshare window is a popup associated with the browser action button. It is a Firefox webextension API. I cannot reproduce the 'wallpaper transition' but most probably it catches with ALL browser action popup.
The context menu is only displayed when you click on images, text selection or page tab.
You can read some tips on how to use the addon here: http://arlogn.github.io/easyshare/