fig, axes = plt.subplots(3, 2, num=0)
# fig.canvas.mpl_connect('key_press_event', on_press)

h, w = image_size
h, w = int(h), int(w)

ax1 = axes[0][0].imshow(np.zeros((h, w, 3), dtype=np.float32))
ax2 = axes[0][1].imshow(np.zeros((h, w, 3), dtype=np.float32))
ax3 = axes[1][0].imshow(np.zeros((h, w, 3), dtype=np.float32))
ax4 = axes[1][1].imshow(np.zeros((h, w, 3), dtype=np.float32))
ax5 = axes[2][0].imshow(np.zeros((h, w), dtype=np.float32)) 
ax6 = axes[2][1].imshow(np.zeros((h, w), dtype=np.float32))

image_np = <(h,w) uint8 or float32>
image_pil = Image.fromarray(image_np).convert('RGB')

...
# ax5.set_data(image_np)  # Fails!
# ax5.set_data(image_pil)  # Works!
plt.imshow(image_np)  # Works!
plt.draw()
plt.pause(1e-9)