Adjusting my thumbnail size

Hi there,

I need to adjust the thumbnail size that my theme is displaying and I don’t know how to do that! I’m running a self hosted wordpress blog at http://www.vellumvoyages.com

Please feel free to visit it and have a look as you can see the book covers are getting cut off due to the size. I think it will have something to do with the CSS? but unsure what else to do to adjust!

Thanks!

VV