Can a user alter sidebar content from the post page?

On my current website (in HTML/CSS/JS), I have an image on my sidebar that changes as the user mouses over links on the content panel. For example, as they mouse over “Writing” the sidebar picture changes to an appropriate writing image. I’m using javascript and mouseover to do this.

I cannot figure a way to do it in WP. I know if the image were in my content page (“post,” I guess) I could use the hover tag and change the y offset of a multiple image jpg (kludgey to the max, btw). But I don’t see a way to change the image that’s in the sidebar.

Help???