Adding the code to the custom CSS section did not fix the issue, unfortunately. Tried deactivating all plugins and clearing all caches too. Still, same problem with small image in upper left corner.
But, I noted the reference to the twentyeleven theme and also the reference to any transition effect except fade in the link you provided. The 2 themes I had installed were yours and twentytwelve. I removed twentytwelve and changed the transition effect to fade. The issue of the small image appearing is now fixed, and without the additional code in the custom CSS section. Plus the other plugins I had are now reactivated. Still works.
I did note the CSS file in the Nivo Slider editor shows as being inactive. Perhaps that has something to do with the other transition effects not working.
But for me, for now, the distraction of the small image appearing in the left corner of the slide is solved. Thanks for that and also your theme, which I’m finding to have a lot of flexibility and to be a great value.