Another way to do what you want is to put the thumbnail into a comment that is tagged to a specific cell. The picture does not show until you mouse over the cell and you can adjust the size as well.

Right click on the cell and choose Insert Comment.
Then Right Click on the border of the comment and choose Format Comment.
In the Format Window choose the "Colors and Lines" tab
In that tab in the "Fill" section click on the "Colors" dropdown box and choose "Fill Effects".
In the "Fill Effects" window choose the "Picture" tab and click on the "Select Picture" button. This will take you to your "My Pictures" folder where hopefully you have stored all the pictures you may want to use. Click on the one you want and then click Insert>Okay>Okay.