Your signature says 4.4.2, yet it's not out yet
Can't you just add banner image as category icons in category admin?
Or, for v4.4.1 (assuming light and lucid skin)
skin/common_files/customer/subcategories.tpl
Insert...
Code:
{if $cat eq "70"}<img src="{$ImagesDir}/banners/70.jpg" />{/if}
Or if you are prepared to make banners for every category...
Code:
<img src="{$ImagesDir}/banners/{$cat}.jpg" />