The editor using for signature or automatic reply email is not support a native function to insert image to the content. So if you need an image in your signature or automatic reply email, you can do a work around as the following:
1. Upload your image to a website, and you can access this image via an URL.
2. Using IE or MS Edge to browse the image via its URL.
3. Right click on the image then select Copy, then paste to the editor by pressing Ctrl + V
If there is another way to do this, please give me a comment here.