Issue with thumbnail overlay

Hi, I have a problem with the highlight box that overlays category and product descriptions.

It doesn't match the thumbnail in size, and I'm not sure what I'm doing wrong.

Category thumbnail

or here
s18.postimage.org/xw9xk64m1/category_thumbnail.gif

Product thumbnail

or here
s15.postimage.org/p7p31qrdn/product_thumbnail.gif

I would like the blue box to completely cover the image (or modify the blue box in any way).

Any help would be appreciated,

Thanks!

Could you please give a link to these pages ?