iPhone / iPad: Reply to Email with embedded image -- Image gets replaced with image.png
This is default behaviour. If you reply, the sender already had the image thus it's not needed to send it over and over again.
If you forward it, you're (most likely) sending it to a different person who did never see the content before, which makes it relevant to add the image.
You cannot change this, except if you find any 3rd party plugin.

If you really want to reply the image, just forward the mail and add the emailaddress of the sender back in the "To" field.