Images in sub categories

[kiwiacw]

I'm wanting to put images in 2 diff sizes for a sub category. On the sub cat page where all are listed I want a 80x80 thumb (square) of an image. Once I click on that sub cat I want a 500x150 image that is full size (ie image takes up all that space not a square with white boders to fill the space if that makes sense)

Any ideas how I can achieve this?

Images are the things causing me the greatest headache at the moment. This for a seed site with hundreds of products and there will be numerous sub cats then products..ie Cat =Flowers, sub cat =Pansy then there could be 20 product vaieties listed the Pansy sub cat

Cheers for any help which is appreciated