You can add a Rectangle
patch to the matplotlib Axes.
For example (using the image from the tutorial here):
import matplotlib.pyplot as plt
import matplotlib.patches as patches
from PIL import Image
import numpy as np
im = np.array(Image.open('stinkbug.png'), dtype=np.uint8)
# Create figure and axes
fig,ax = plt.subplots(1)
# Display the image
ax.imshow(im)
# Create a Rectangle patch
rect = patches.Rectangle((50,100),40,30,linewidth=1,edgecolor='r',facecolor='none')
# Add the patch to the Axes
ax.add_patch(rect)
plt.show()