resize images using % in css

I am trying to create a liquid web layout using % for as many things as i can. I have hit a bump when resizing images.

both:

<img src="source" style="width: 20%; height: 20%;"/>

and

.wall_picture_block img{
width: 20%; 
height: 20%;
}

don't work properly with the height attribte. They resize the image width to 20% of the container but the height stays relative to the image size.(im trying to make squares)