First of all, it's not a WordPress related you know and there are other platform you can ask about this one,
anyway, if you didn't read this http://inspirationalpixels.com/tutorials/creating-a-dropdown-menu-with-html-css kind of article please read one.
I see you changed your post summary and thanks for that.
But before get answer something worth you have to try something because your solution can not be done with small code/example. It's need couple to step, if you want do it by your self.
- Need to upload field in WordPress Menu Page
- Working with Walker_Nav_Menu according to your styles and upload
- use your custom menu function as walker hook.
But if you want to use use any mega menu plugin that will be easy and perfect for this.