Need clarity on the header images

website: http://www.chelmsfordsaicenter.org.

On the website, i have a slider for the header image. I have added 2 images on each slider. The images are not clear. I had to shorten the size of the image. When i shorten the size, the clarity is gone. It looks blurry. Can you suggest what i can do to get the clarity of the images in the slider?