What I am doing wrong ?

File "D:\Python\programs\IMG PRO\im.py", line 30, in <module>
    threshhold(iar)
File "D:\Python\programs\IMG PRO\im.py", line 17, in threshhold
    echpix[0]=255
ValueError: assignment destination is read-only

from PIL import Image
import numpy as np
import matplotlib.pyplot as plt
import time
def threshhold(imagearray):
    newa=imagearray
    bal=list()
    for echrow in imagearray:
        for echpix in echrow:
            avg=sum(echpix[:3])/3
            bal.append(avg)
    s=sum(bal[0:])
    balance=s/len(bal)
    for echrow in newa:
        for echpix in echrow:
            if (sum(echpix[:3])/3)>balance:
                echpix[0]=255
                echpix[1]=255
                echpix[2]=255
                echpix[3]=255
            else:
                echpix[0]=0
                echpix[1]=0
                echpix[2]=0
                echpix[3]=255
    print newa

im=Image.open('../images/numbers/y0.3.png')
iar=np.asarray(im)
threshhold(iar)
fig=plt.figure()
ax1=plt.subplot2grid((8,6),(0,0),rowspan=4,colspan=3)
ax1.imshow(iar)
plt.show()