What I would like is to use specified functions or lines to create the border of the images, and then inside the borders place custom images.
I used fill between as an example, but maybe there is an easier way of achieving this. This is what I have in mind:
import matplotlib.pyplot as plt
import numpy as np
x = np.linspace(0,100,1000)
y = (3 + np.sin(x/5.)) * np.exp(-abs(x-50)/25.)*10
x2 = np.linspace(20,40,1000)
y2 = np.sqrt(100 - (x2 - 30)**2) + 40
y3 = -np.sqrt(100 - (x2 - 30)**2) + 40
plt.figure()
ax1 = plt.subplot(1,1,1, aspect='equal')
plt.fill_between(x=x, y1=y, y2=0, alpha=0.5, color='orange', linewidth=3)
plt.fill_between(x=x2, y1=y2, y2=y3, alpha=0.5, color='green', linewidth=3)
ax1.axis('off')
the output of that code is this image, what I have:

and I would like something like this, what I want:

Please have in mind that I would like to have a border around the image that I can customize it, and if I have more than one fillbetween, I can choose what to put inside the different curves.
And also if its possible to respect the axis ratio of the image so its shape doesn't get distorted when I place it in the plot.