Тестирование функции,
с использованием Python 2.7.13, Ipython 5.1.0, skimage 0.13.0,
и Python 3.6.7, Ipython 7.4.0, skimage 0.15.0:
In [1]: from skimage import io
In [2]: a = io.imread('testimg.tif')
In [3]: type(a)
Out[3]: numpy.ndarray
Ваша ссылка на документацию - Skimage 0.16.0, но я думаю, можно с уверенностью предположить, что в документации просто есть опечатка.
edit: также, смотряу источника :
def imread(fname, as_gray=False, plugin=None, flatten=None,
**plugin_args):
"""Load an image from file.
Parameters
----------
fname : string
Image file name, e.g. ``test.jpg`` or URL.
as_gray : bool, optional
If True, convert color images to gray-scale (64-bit floats).
Images that are already in gray-scale format are not converted.
plugin : str, optional
Name of plugin to use. By default, the different plugins are
tried (starting with imageio) until a suitable
candidate is found. If not given and fname is a tiff file, the
tifffile plugin will be used.
Other Parameters
----------------
plugin_args : keywords
Passed to the given plugin.
flatten : bool
Backward compatible keyword, superseded by `as_gray`.
Returns
-------
img_array : ndarray
The different color bands/channels are stored in the
third dimension, such that a gray-image is MxN, an
RGB-image MxNx3 and an RGBA-image MxNx4.