[pdftex] Image transparency

[Michael Chapman]

Apologies in advance, this sounds so basic I fear it may be an "oldie".

Am converting a HTML page to a PDF brochure.
Using pdflatex with pdfscreen.
Marvelous.

Only problem I have converted various GIFs to PNGs and am pretty sure I have 
conserved their transparency.
However viewed against the page backgrounds that pdfscreen uses, they come 
out on a white local area (i.e. the size of the image).

Doing something wrong, or just asking too much ?

Michael Chapman.