Customise plot_image()

I would like to customise the plot created by the plot_image() function by adding an ax.axvspan on the evoked response where the p-value (obtained with permutation_t_test) is smaller than 0.05.
How can I access the source code of the function to understand how to do that ? Or if you can explain me how I can add this without changing the initial grid/organisation of the plot.

Thanks a lot