different size images for each category

Hi,

I suppose that you're talking about the thumbnails on the listings of products.
Usually, when you change the size of the images for the listings, you also want to change the number of elements displayed on each row (to have more space, etc), or other settings.
In that case, you should create a new menu item below the menu item of the main category for that sub category. The system should then automatically pickup the URL of the sub menu item for the link of the sub category on the listing of the categories so that when you arrive on that sub category listing of products, it will use the settings of that sub menu item instead of the settings of the main menu item for the display of the products. And thus, you have all the leeway you want to customized the way the products listings are displayed for different categories, including changing the size of the thumbnails thanks to the "image width" and "image height" settings under the "products options" tab of the menu items.