Duena Theme. Having problems with featured image and post image

I am running Wodpress 4.0 and the Duena Theme. I am having a major issue with my images. I write my post and then want to add a image to the post so you can see it without having to click on the actual post. I do this by setting a featured image. When I do this the featured image is HUGE, and the post image is blurry, cut off, you name it. Please help me as I am new to all of this and I have no clue what I’m doing. You can see my blog here kempnitreal.com. The images are just too large in the display! Please help me.