Have iTerm use a random background for each new window. See https://iterm2.com/python-api/ for details.

iterm-random-background.py

#!/usr/bin/env python3.7

# Be sure "Separate background images per pane" is unchecked.

import asyncio
import iterm2
import json
import os
import random

window_backgrounds = {}

async def main(connection):
    app = await iterm2.async_get_app(connection)
    async with iterm2.LayoutChangeMonitor(connection) as mon:
        while True:
            await mon.async_get()
            for window in app.windows:
                background_path = get_background_for_window(window)
                await set_session_background_paths(window, background_path)
            purge_closed_windows(app)

def get_random_background_path():
    backgrounds_dir = "/Users/chris/Pictures/Terminal Backgrounds"
    random_background_file = random.choice(os.listdir(backgrounds_dir))
    return os.path.join(backgrounds_dir, random_background_file)

async def set_background(profile, path):
    await profile.async_set_background_image_location(path)
    await set_background_settings(profile)

async def set_background_settings(profile):
    await profile.async_set_blend(0.25)
    await profile.async_set_background_image_mode(2)

def get_background_for_window(window):
    global window_backgrounds;
    if window.window_id not in window_backgrounds:
        window_backgrounds[window.window_id] = get_random_background_path()
        print(f"New background chosen for window {window.window_id}")
        print(json.dumps(window_backgrounds, indent=2))
    return window_backgrounds[window.window_id]

async def set_session_background_paths(window, background_path):
    for tab in window.tabs:
        for session in tab.sessions:
            profile = await session.async_get_profile()
            if not profile:
                continue
            if profile.background_image_location != background_path:
                await set_background(profile, background_path)

def purge_closed_windows(app):
    global window_backgrounds
    windows_to_purge = []
    for window_id in window_backgrounds.keys():
        if not app.get_window_by_id(window_id):
            print(f"Window {window_id} has been closed. Purging.")
            windows_to_purge.append(window_id)
    for window_id in windows_to_purge:
        del window_backgrounds[window_id]

iterm2.run_forever(main)