Here is another way you could do it. This will work if all teh logo images are the same size
1) create a blank logo as a 'png' call it blank_logo.png and put it in the atahualpa/images folder
2) at ATO->Style & edit HEADER AREA->Show Logo Image? enter 'blank_logo.png
now the fun part
at ATO->Add HTML/CSS Inserts->CSS Inserts add the following
HTML Code:
body img.logo {
background:url(http://yourdomain.com/wordpress/wp-content/themes/atahualpa/images/main_logo.jpg);
}
body.category-announcements img.logo {
background:url(http://yourdomain.com/wordpress/wp-content/themes/atahualpa349/images/cat-announcement-logo.jpg);
}
just repeat the second CSS Selector/rule for the categories you want to deal with.
What this does is first display the blank logo, then on the main page it overlays the 'main_logo' except on the category-announcements page. there it will show the cat-announcement-logo.jpg.
The name of the categories will be what ever you are calling your categories.