Mantra : image near title in columns and # slider

Hello everyone,

I need your help to customize Mantra theme :
– i’d like to add an background image to the #slider. In which file can I do it, this id is not in the style.css file but under…/#6
– I’d like also to get the column-image, small, on the left size of the <h3>title.
Thanks for your help guys !
Marine