Problem displaying images in sub-folders

I’ve encountered a bizarre problem that I can only replicate in Firefox and IE. If I use this for for images:

<img src=”<?php bloginfo(‘stylesheet_directory’); ?>/images/ads/ad-example.gif” alt=”advert” width=”370″ height=”60″ />

It won’t display the image because the image is in a sub folder, however if I use:

<img src=”<?php bloginfo(‘stylesheet_directory’); ?>/images/ad-example.gif” alt=”advert” width=”370″ height=”60″ />

The images work fine, obviously I can move all files to the images folder but sometimes I need to have images in sub folders.

Strange problem as it’s only just happened?