Code:
{* NO sc.thumb_utl VARIABLE IS BEING FOUND SO WE NEVER GET INTO THIS IF STATEMENT *}
{if $config.Reboot.reboot_mega_icons eq "Y" && $sc.thumb_url}
<p class="megamenu-icon"><a href="home.php?cat={$sc.categoryid}"><img src="{if $sc.thumb_url}{$sc.thumb_url|amp}{else}default_icon.gif{/if}" width="{$sc.image_x}" height="{$sc.image_y}" alt="{$sc.category}"></a></p>
{* SINCE NO sc.thumb_url VARIABLE IS DEFINED, WE GET INTO HERE, BUT WE KNOW THERE IS AN IMAGE *}
{elseif $config.Reboot.reboot_mega_icons eq "Y" && !$sc.thumb_url}
{* REPLACED THIS DEFAULT CODE TO JUST FIND THE CATEGORY URL USING THE get_category_image_url FUNCTION
<p class="megamenu-icon"><a href="home.php?cat={$sc.categoryid}"><img src="default_icon.gif" alt="{$sc.category}"></a></p>
*}
<p class="megamenu-icon"><a href="home.php?cat={$sc.categoryid}"><img src="{get_category_image_url category=$sc}" width="100px" height="100px" alt="{$sc.category}"></a></p>
{/if}
Here is what we did