What sort of image are you trying to ouput? If you grab the image data as an multidimensional array it's easy to convert images.
For example, in python setup for array processing (for easy setup use neurodebian, pythonxy, or enthought python with pynifti installed) you can do something like this:
from numpy import *
from pyplot import *
import nifti
nimage32 = nifti.NiftiImage('my32bit.nii.gz')
arr32 = nimage32.asarray()
arr8 = zeros(arr32.shape, dtype='uint8') # create an 8 bit array to hold the 8 bit image values
a, b = arr32.min(), arr32.max()
c = b-a
arr8[:] = 255*(arr32[:]-a)/c # scale image data so values filll 0-255
nimage8 = nifti.NiftiImage(arr8) # create a new nifti format image with 8bit pixels
# if you want to save something as a regular image like a png or in pdf you can slice the array to pick out a single slice and save it
imsave('single.png', arr8[0,0,:,:], cmap=cm.gray) # from pyplot, use first volume, first slice, save as png file
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