Image Resolution in Slider Section

The default images in the slider section seem to be in a better resolution than what my images appear to be, even at the suggested size of 1920*960 pixels. How do I change those settings so my own images appear as vibrant and crisp as the default images that were initially provided? What are the recommended resolution settings for the size? What image types are accepted or suggested?

My page is still a work in progress, so a lot of it is still set to default view…

The page I need help with: [log in to see the link]