Just change the background size within the original "if" value.
Put background-size:0%
<div class="content_scene_cat_bg" {if $category->id_image}style="background:url({$link->getCatImageLink($category->link_rewrite, $category->id_image, 'category_default')|escape:'html':'UTF-8'}) 0 top no-repeat; background-size:contain; min-height:{$categorySize.height}px;" {/if}>
The image will then disappear from the category header but will remain as a thumbnail when browsing your menu.