Static image excludes layout_images if source is not absolute path

[tillhanke]

Hi there, I am currently failing to export a static image including a layout image.
The export does not show any errors but the layout images are just omitted in the exported data.
This is the case for fig.write_image() as well as plotly.io.to_image().
A simple example is:

import plotly.graph_objects as go
import numpy as np

x = np.linspace(0,3*np.pi,1000)
y = np.sin(x)

plotly_logo="Images/plotly.png"
fig = go.Figure(go.Scatter(x=x,y=y))
fig.add_layout_image(dict(
    source=plotly_logo,
    xref="paper",
    yref="paper",
    x=.5,
    y=.5,
    xanchor="center",
    yanchor="middle",
    sizex=.4,
    sizey=.4
))
fig.write_image("Images/test.png", scale=3)

The image will be saved and show the sine curve but the plotly logo will not be visible.
But fig.show() shows the plotly logo nicely centered in the graph.

Switching out the relative path Images/plotly.png with a absolute one e.g. /home/user/Images/plotly.png the logo appears in the exported image file as well.
For me it seems, that the source argument is interpreted differently for different methods of showing the image.
Maybe someone knows, where to look to fix this?