Replace
<%= image_tag("my_image.png") %>
With
<%= image_tag "http://mydomain.com/path/to/my_image.png" %>
For example:
<%= image_tag "https://www.google.com/images/srpr/logo11w.png" %>
The above example would add Google Logo in your email body.
If you want your own image to be shown then replace
mydomain.com
: Your domain name
/path/to
: Path to your folder where my_image.png
resides