Nadia - you are using backslashes in your string which is probably generating the following warnings
Warning: Control Character '\i' is not valid. See 'doc sprintf' for control characters valid in the format string.
Instead, build the filename separately from the path, and then use fullfile to build the full file name (with path) as pathName = 'F:\b';
for k=1:4
b = zeros(32,32);
fileName = fullfile(pathName,sprintf('i%d.bmp',k));
imwrite(b,fileName);
end
Your file names will look something like
F:\b\i1.bmp
F:\b\i2.bmp
F:\b\i3.bmp
F:\b\i4.bmp
Note that I replaced your index of i with k since i (and j) are also used by MATLAB to represent the imaginary number. I also removed the bmp input argument to imwrite since the image file extension has already been specified in the file name.