Optimizing Picture for Slider (icraft theme)

I love this theme and have it up and working with Woocommerce. The only thing i don’t like is the quality of the pictures in the slider. When i upload the pics they are cropped and the quality takes a hit. How can i optimize my pictures to fully fill in the slider and keep the quality? Is there a certain dimension they need to be (pixels LxW) to fit the slider and not corrupt the quality of the picture. And what tool can i use to optimize the picture? Photoshop etc? Thank you.