So, here's what I did:
1. I copied the image from Tineke's blog post: Hexagrams from the Yijing (Book of Changes) randomized. Sixty-four hexagrams, from a book owned by Leibniz. Source: Wikipedia. Here is the image:
https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEge3ScPJMN4XuY8Su11rA2h7_Mn7cSl3uKv4qoJ6-GibsK1PX0idrs9vncMHtLZGC68it6guu19jpJN3mUoBnxEyw_N88h-D0j7L1uygOy5_bZzY0oSe4vcYcoLjT7Sunz15mGjVUyX4UE3/s400/Diagram_of_I_Ching_hexagrams_owned_by_Gottfried_Wilhelm_Leibniz_1701-1024x984.jpg
3. Then I went to the Layout in my Dashboard, and clicked to add HTML/Javascript.
4. In that box, I pasted in Tineke's javascript code, centered. Then I added a bit of text to alert people to click for the English. Then I added the image using the URL above for the img src, and setting the width to 200 pixels. It worked! So, that proved my guess that it is possible to publish an image in a blog post and then reference that image in the sidebar with the URL. I wasn't sure if it would work, but it did!
Now I can go add this to my HTML Box post where I had promised writing up something like this; thanks to Tineke, this was a perfect little experiment to try!