When a spoiler blockquote contains text, the text transitions in smoothly using a CSS transition. If it contains an image, the transition is abrupt.

[I've shared a brief video of the transition as-is][1]

If the CSS were changed from:

    .spoiler:hover img {
        visibility: visible;
    }
    .spoiler img {
        visibility: hidden;
    }

to:

    .spoiler:hover img {
        opacity: 1;
    }
    .spoiler img {
        opacity: 0;
        transition: opacity 0.5s ease-in;
    }

[The image transition would be much smoother][2].

[1]: http://screencast.com/t/xXISf1jW
[2]: http://screencast.com/t/zqMOdZrHstmn