What are you wanting to edit? If the picture itself, then you need to do that with what ever photo editor you choose then save the edited pic or video.
I many times can just drag a pic from the web to the reply window, whether I'm in the quick reply or the advanced editor. Sometimes certain websites I try to grab a pic from I'll have to use the editor tools or attachment manager.
If you are just wanting to add a comment... well do that in the message editor if the attachment manager doesn't give you that chance.