Featured image

My featured images looked great in the slider but looked very blurry above each blog post. This occurred on both the home page and the post pages. I have had to switch themes temporarily until I can figure out how to prevent the blurriness. It is almost like a thumbnail has been blown up to a large size. The images that I included look great in the slider, look great in the posts and look awful as the featured image above the post excerpt. Can you help? I would like to go back to your theme?

I will sneak in one other quick question. Is there a way for me to specify the images on the slider instead of having it based on my posts?