By default, Super Hero Slider will use large size images. You can see what size your large images are by going to Settings > Media in your dashboard.
You can also create a specific size for your slider images by going to Settings > Super Hero Slider and adjusting the ‘Image width’ field.
You can then specify the ‘Default image size’ to use in your slides.
If you experience pixellation with your image quality, we recommend increasing the image width. Note that you may have to run a plugin that will resize your images. A good example is Regenerate Thumbnails.