403 error with img tag in HTML ad. BUG ?

Hi !

If I create an HTML ad with this HTML code in it :

<img  src="http://www.douance.org/douance-enfant-surdoue-livre-anim.gif" width="120px"  height="600px">

when I try to save it, I get a 403 error page.

If I remove the "." in the URL (like src="http://www.douance.org/douance-enfant-surdoue-livre-animgif") I don't get the 403 error and the ad saves itself correctly.

Why I am getting this 403 error if there is a dot in the src URL ??