Assuming that that is really what you have called your images, what about something like this:
<%if Category_Image%>
<img src="<%build_images_url%>/<%Category_Image%>" border=0 />
<%elsif FatherID > 0%>
<img src="<%build_images_url%>/categoryheader_<%FatherID%>.jpg" border=0 />
<%endif%>
<%if Category_Image%>
<img src="<%build_images_url%>/<%Category_Image%>" border=0 />
<%elsif FatherID > 0%>
<img src="<%build_images_url%>/categoryheader_<%FatherID%>.jpg" border=0 />
<%endif%>
