Header Image CSS? – Noobie question

I am running BP 2.1.9 and am customizing the GROUP header area with the style.css

I can successfully edit the height of the image here:

div#item-header img.avatar {
		float: left;
		margin: 0 15px 25px 0;
		height: 30px

	}

around linne 335, but as soon as I put a width field, both the height and the width default back to normal (as if they were not there)

I am new to css, so this may be a very simple answer. Any thoughts?