I need help with tumblr coding!

I just set up a tumblr account, and have a few cool things in mind that I want to add to my tumblr page. I DO have some HTML coding experience, but not much..

anyways, what I want is a link that is a picture of a skull. But when you move your mouse over it, the skulls head will open. I already have both skulls drawn, but I just don't know how to code it in that way.
I know how to make a picture into a link using coding; I just don't know how to make it so that when you run your mouse over the image, It will change.

Does this make any sense at all?? XD
Please help? :')

I have no more idea about webdesign but still before some days my friends Dibesh had made a website for his organization from www.helveticbrands.ch. I think they help you better.