image/PC

Hi all,
I am using the psychtoolbox with windows 98 and encounter problems
while presenting an image. It's a simple white on black background
texture. However, trying to display it with 'PutImage' after reading
it into MatLab gives me always the reversed pattern, e.g. black on
white background.
I tried the uint8 class and the double as well (double(X)*255). Is
there any way to force the PC to display 255 as black and 0 as white.
'SetClut' and chaging the White/BlackIndex files didn't work.
Thanks for any help,
Bjoern