How to set a fixed size for all images

Hello,
Well here is my problem.
I have a group of images

<div id="slider">
    <img src="images/ScalesEmulator.png"/>
    <img src="images/discussing.jpg" />
    <img src="images/four-main-steps-for-a-software-process-cycle.jpg"/>
    <img src="images/globe.jpg"/>
    <img src="images/shake-hands.jpg" />
</div>

styled with:

#slider {
	float: left;
	height:320px;
	width:650px;
}

#slider img {
	width:650px;
	height:320px;	
}

There is some JavaScript code which will make them keep changing between themselves. The problem is that images might be either smaller or bigger than the size I set on css and for some reason they won't fill the space it was set to them.
So far I couldn't find a way to do this. If I am required to change every picture I have to place on the site, I will do it. But I would like the opinion of people that have more experience with website about my options.

Thanks

Tags: CSS, HTML, Web Development