Styling Product page, characteristics and cart

Hi!

I'm having some trouble finding out where i can change the characteristics layout.
I have a characteristic called size, and i would like to display it on the product page like this

SIZE <img src"my image" /> DROPDOWN(and i would like to change the background of de dropdown as well)

i can´t figure out which php and css files should i edit...

Also i would like to separate the item quantity from the add to cart option, want to put them in different lines, which file should i edit?

I have the hikashop essentials.

Miguel