How do I override the "This image has been removed" message?
First, you need to know which input text filter you have enabled where the images are not showing. For the sake of this answer, let's say it's the "Basic HTML" input filter. 

Go to the configuration for this input filter, such as /admin/config/content/formats/manage/basic_html 

Scroll down to "Enabled Filters" and look for:

"Restrict images to this site"
Disallows usage of  tag sources that are not hosted on this site by replacing them with a placeholder image. 



If you can not view externally-hosted images, it is likely this filter is enabled. Simply disable this filter.