Draw things naturally onto an image using matplotlib

figimage.py

import matplotlib.image as mpimg
import matplotlib.pyplot as plt

def loadFigImage(path):
    im = mpimg.imread(path)
    h,w = im.shape[0], im.shape[1]
    dpi = 80
    fig = plt.figure(figsize=(w/dpi, h/dpi), dpi=dpi)
    ax = plt.Axes(fig, [0, 0, 1, 1])
    ax.set_xlim(0, w)
    ax.set_ylim(0, h)
    ax.invert_yaxis()
    ax.set_axis_off()
    fig.add_axes(ax)
    fig.figimage(im)
    return fig, ax

def saveFigImage(path, fig):
    fig.savefig(path, dpi=80)
    plt.close(fig)
    
if __name__ == '__main__':
    import numpy as np
    fig, ax = loadFigImage('image.png')
    # plot using image coordinates, (0,0) = top left
    x = range(0, 2000, 50)
    y = np.random.randint(0,1000, len(x))
    ax.plot(x,y, c='red')
    saveFigImage('image2.png', fig)