Try this in your css:
img {
max-width:100%;
max-height:100%;
width:100%
}
in the html:
<div><img alt="" src="..." /></div>
That will make ALL your images fill the width to the max extent possible while maintaining the correct aspect ratio.
If you have specific images that you want to behave differently, you can give them a specific class.
e.g.:
img.half {width:50%}
<div><img class="half" alt="" src="..." /></div>
Cheers!