Responsive Images with CSS

I'm finding it tricky to resize images to make them responsive.

I'm developing a php application to automatically convert a website to a responsive version. I'm a little stuck on the images.

I've successfully added a wrapper class to every image on a website and can re-size the images quite well.

My issue lies with images that are naturally smaller than the the window, such as logos and icons. I don't want to resize these.

My code currently converts:

<img src="[src]" />

into:

<div class="erb-image-wrapper">
    <img src="[src]" />
</div>

Where I use the following CSS:

.erb-image-wrapper{
    max-width:90%;
    height:auto;
    position: relative;
    display:block;
    margin:0 auto;
}
.erb-image-wrapper img{
    width:100% !important;
    height:100% !important;
    display:block;
}

This resizes all images, but I only want it to resize images that are over the width of the page. Is the a way I can achieve this via CSS?