sure you can use smarty and set if conditions to show any image you like.
for example.
{if $cat=22}<img src="{$ImagesDir}/yourcategory22image.jpg" alt="" />
{elseif $cat=23}<img src="{$ImagesDir}/yourcategory23image.jpg" alt="" />
{elseif $cat=24}<img src="{$ImagesDir}/yourcategory23image.jpg" alt="" />
{else}defaultpic.jpg
{/if}
This would go in teh head.tpl wherever you want the image to apear.
upload the iamges to your skin1/images folder.