Position child elements in Container
===

I am trying this:
there is a picture element Which is wrapped inside a container div 

    `<div id="container">
        
    
        <div id="a">
            <img src="b.jpg" alt="b" />
        </div>
        
    </div>`
Now I want to place the child elements such a way that the top and left of the picture is always 25% of the height & width of the container div. How can I achieve this?