Wordpress Blog to be set up correctly to work within my site

Hi.

I currently have a wordpress site and use the default blog.
when create a new blog and set the featured image, no matter what size image I use, it either shows up as massive or it crops the image.

I need the blog set up so I can simply add a image to the featured image part of the blog page and it will simply fit into the thumb nail and mane image correctly

the website is www.copperfrogdistilling.co.uk

Cheers
Simon