Are you overwriting the image found here:
- ads /modules/boonex/ads/templates/base/images/icons/bx_ads.png?
Here's the bit about the submenu header icons in general from image tutorial, it might help. I had no problem changing out the ads icon using this (just remember that if you don't overwrite the existing file and you want to use a different .png image, you still have to have it in either the /modules/boonex/ads/templates/base/images/icons/ or the /modules/boonex/ads/templates/<yourtemplate>/images/icons/ folder in order to set it correctly in the Navigation Menu box):
These are the images in the submenu header on each internal page of the site. They are 32x32 by default, but this is one of the places where the formatting will adjust to the size of your image. As near as I can tell from playing with it, you can go as large as 64x64 with the submenu icons before the formatting of your page body is distorted and pushed down lower. I make no guarantees about that, though, ymmv! 
To replace this image, go to Admin panel>Builders>Navigation Menu and click on the name of the top level menu name of the page that you are wanting. Change it in the box labeled "Picture:". Save all changes. If you overwrite existing images through uploads, remember to follow the same path but into your template folder instead of base. For example /templates/base/images/icons/ becomes /templates/your_template/images/icons/.
