@electricidee: It would have been easy for me to answer if you have posted your site URL. Looks like you are using Catch Box Free theme. If you see the theme instruction of Catch Box http://catchthemes.com/theme-instructions/catch-box/, it’s mentioned there that the best size for the featured slider is Width: 644px and Height: 320px.
So, the best use will be adding the featured image of Width: 644px and Height: 320px and you will have all image same. If the image is bigger then the size mentioned it will automatically crop the size but if it is smaller then it will just show the smaller image.
Another, is that if you have just change your theme from other theme then you need to regenerate the thumbnail.
.
