'**SUMMARY**
Return an np.array that are the width and height of every image in the image set.
**PARAMETERS**
--NONE--
**RETURNS**
A 2xN numpy array where N is the number of images in the set. The first column is
the width, and the second collumn is the height.
**EXAMPLE**
>>> iset = ImageSet("./b/")
>>> sz = iset.dimensions()
>>> np.max(sz[:,0]) # returns the largest width in the set.'

def dimensions(self):

    retVal = []
    for i in self:
        retVal.append((i.width, i.height))
    return np.array(retVal)
