[BCSE]

I haven't tested this so I'm not 100% sure, but go to categories.tpl (under skin1/customer)

Insert the image infront of

<font class=CategoriesList>{ $categories[cat_num].category_name|escape }</font>

and

<font class=CategoriesList>{ $subcategories[cat_num].category_name|escape }</font>

Do the 2nd one only if you want it to show up beside sub categories too.

I'd probably put the image in skin1/images and reference it like
[img]{$ImagesDir}/your_image.gif[/img]

HTH

Carrie