[SOLVED] Getting single posts to inherit header for their category

I have created two virtual blogs from a single install, using two basic categories and a menu link to each category. Now I'd like to have a different header image (banner) for each virtual blog -- i.e., I'd like posts to show a different header image depending on whether they belonging to one basic category or the other.

I can get the effect I want for the category pages using the following CSS:

body.category-1 div#imagecontainer {
background-image: url(/wp-content/ata-images/banner-1.jpg) !important;}

body.category-2 div#imagecontainer {
background-image: url(/wp-content/ata-images/banner-2.jpg) !important;}

However, this doesn't work for single post pages, which revert to the default site banner. Is there a simple way I can get single post pages to inherit the header image assigned to the category to which they belong?

I'd be very grateful for any help! Thanks!

Best, Kit