There is "category icon" for each category. Upload your image there for each category and then just change the code for the header image to read
(code taken from subcategories.tpl)
Code:
<img class="subcategory-image" src="{$current_category.icon_url|amp}" alt="{$current_category.category|escape}"{if $current_category.image_x} width="{$current_category.image_x}"{/if}{if $current_category.image_y} height="{$current_category.image_y}"{/if} />
or this (code taken from subcategories_t.tpl)
Code:
<img src="{$subcategory.icon_url|amp}" alt="{$subcategory.category|escape}" width="{$subcategory.image_x}" height="{$subcategory.image_y}" />
whichever works