Problem with imagedata

[jbv]

I am trying to modify the imagedata of an image
one pixel at a time.
The image is totally filled with black pixels.
In a variable (whitepix) I stored the 4 binary
values of a white pixel. When I run the following
piece of script, the WHOLE image becomes white,
not only the requested pixel :

get imagedata of img 1
repeat with i=1 to 4
    put char i of whitepix into char (i+131400) of it
end repeat
set imagedata of img 1 to it

what did I do wrong ?

Thanks,
JB