to get this

1. Select the cell, and "right button click"

2. Choose "Insert Comment"

3. After clicking on "Insert comment" the comment box will open up and look like this

4. Normally, all you want to do is just type in some words and then you'll get a "comment hover" when the mouse goes over the box, BUT, you can also get an image to hover when the mouse goes over the box.

So what you have to do is
  4A - click once in the box, this changes the border from a single skinny line to a dotted line
  4B - then move the mouse down so it just crosses the line of the box, and "right button click" so the second box pops up

5. Select "Format Comment"

6. Click on "Format Comment", and this box will show up giving many different options

7. In this "format" box there is a tab for "Colors and Lines"
Click on that tab, and the box below will show up

8. Now, there is no word saying "Image", what you have to do is click on the little arrow beside the word "Color"
- as in this screen capture

9. When you click on this arrow, a sub-menu pops up, like the screen capture below

10. Move your mouse down to where it says "fill effects" and click on that

11. Now.... yet another box pops up...

12. This new box has 4 tabs at the top

Usually the "Gradient" tab is selected

13. To put a picture in the box select,...... duh.... "Picture", and click on that tab.
After you do this, this is what shows up

14. Below the tab "Picture", is a tab that says "Select Picture"
 

Once you see the "Select Picture" tab, you have to click on that to see the path to get the picture

15. After you click on "Select Picture" what happens is that it automatically launches the window
showing you all the images you have in your Windows "My Pictures" folder

If the image you want to stick in the "hover" is in your "My Pictures", just select it,
if not, then click around to whatever other directory you have the picture in, select it

16. So then Click on the picture
(in my case, I selected a pic of a jeep)

You'll see a border go solid around the image, once you clicked on it

17. Then click on the "insert tab"

18. When you click on the "insert tab" it automatically jumps the image into a preview box

If you "t'ink e'ry ting copasetic", or if you think everything looks OK, just click OK

19. After you click on "OK", you'll see the image jump into the "Color:" box

20. Then you click on "OK" one more time

21. And.... as Ali G says "Booyakasha" - you're finished, and it should look like this