Slash problem in link

I am having a weird problem. I am not sure if this is a php problme or plain html problme but see if anyone can help. I have a page called login.php. It has the following image link. It works at home fine when i put the link as below <IMG src="images/onlylogo.gif"> We tried putting the same page on another customer and it wont show. We put a slash in the front of image and it worked and we can view the image. <IMG src="/images/onlylogo.gif"> But the slash does not work at home on my machine. When do we have to use the slash and when not ...how can we tell please help