How to remove dependence on initial installation path?

[borus]

Hello.

I installed XAMPP at my usb stick #1 trying to create a portable webserver. Then I deployed Joomla site there
I was trying to embed the watermark overlay, so I added the line of code
$wmfname = JPATH_ROOT.DS.'watermark.png'
in some joomla file

When I copied this XAMPP to another usb stick #2 I've found that site looks for watermark picture with the path of the first usb stick.

Please tell, how can I make the watermark path definition to be independent from a usb stick disk letter? Is there any XAMPP settings/configurations which make launched sites independent from the path where the XAMPP was initialy installed? Makes it portable in other words

Thank you