In the skin's config.xml, find the <image reference="category">. It is set for 800 pixels on the longest dimension.
Change this to 900. It will auto-scale down to fit the container (which is 848 px wide).
Next, you will need to find the CSS rule (I do not know where it would be.):
.cat-hdr-img img {
margin: 0px auto 5px;
}
Change to:
.cat-hdr-img img {
margin: 0 0 5px;
}
This CSS change will remove the margin that the browser adds to either side of the image, in equal part, to center the image in the slightly wider container.
Be sure to clear Cubecart's cache.