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Why some images can be load in background while others cannot?

Hi, there. I am going to show image in background. Here is my code.

import plotly
from plotly.graph_objs import *

plotly.offline.plot(
    Figure(
        data=Data([Scatter(),Scatter()])
        ,layout=Layout(
            images=[dict(
                #source='https://cdn1.epicgames.com/ue/item/Store_MarsLandscape_screenshot_3-1920x1080-99ddae9cde7ea6a36f2eef2cc8e2b944.png',   #1
                source='http://i.imgur.com/A3DSgi0.png',    #2
                layer="below",
                sizex=50,
                sizey=50,
                x=-25,
                y=25,
                xref="x",
                yref="y"
                )
            ]
        )
    )
)

If I use #2, I can see the image in web page (chrome).
But if I use #1, I cannot see the image. Why?

By the way, I download A3DSgi0.png and share the image by using http.server.
python3 -m http.server
And change #2 to
source='http://localhost:8000/A3DSgi0.png'
But plotly cannot load the image in background too.