Are you talking about inline images?
Those should be preserved when you're editing your response (in the area below) and sent as such....
.... unless you've used the "pencil" icon on the right of the panel separating new and old content (in the reply)
... in which case the two (new and old content) will be merged together as plain text.
In this case, the images are lost, and you'll see whatever text/plain content there was in the original message.
It's conceivable that some mail apps would put something like "[image: a kissing frog.jpg]" in text/plain content at the same spot where an the actual image was in the text/html content.
Does this explain what you're seeing?
