It sounds like you already have a pretty decent idea of what you need. Use css for this.
For instance:
<img src="http://www.example.com/image.jpg" class="image_class"/> <style> .image_class{ width: auto; height:330px; </style>
You want to target based on the class (and not all images using img{...} ). I have not used WP Slider, but I assume that it will add a custom class to the images in the slider. If so, just use your developer tools in a browser or view source to find out which class is being added to these images, and then create css as described above.
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