An even easier to use plugin for misquotes and testimonials!
Unless you specify the quote by its id, it'll be randomly selected.
That's because you're not only associating the quote with a featured image, you're adding the image to the quote itself. If you click 'Use as Featured Image' at the bottom of the image properties dialog, then 'X' out of the dialog, you'll have a featured image without inserting it into the quote.
While this is a bit counter-intuitive, it gives you the power to insert images and other rich content into quotes when you actually want to do that.