Post thumbnails small then big! Please help with featured image

Hi there,

I’m currently running on 3.1.3 with a custom theme designed with Artisteer. When I add a featured image to my posts using the ‘featured image’ option, it works beautifully and when displayed as a list of blog posts, all the little thumbnails are there.

However when you click through onto the actual posts themselves, the images are massive and not thumbnail size at all. Do you know how to remedy this issue?

Please make it as simple as possible as I am no code monkey. Is there a plugin that will help?

thank you,

ian