You're encountering a confusion that has come up before.. The problem is that Ctrl+C can do a different thing depending on where you are.
Here's what I think happens, perhaps you can verify:
If you hit Ctrl+C while in the main window, you should copy the IMAGE to the clipboard, and pasting it in your email will paste inline.
BUT if you are in the lefthand thumbnail panel, when you hit Ctrl+C you are copying the FILE to the clipboard, and pasting it will be like dropping a file into your email, as an attachment.
Does that make sense? I could change it, but there is some logic to the way it works.
So if you want to copy and paste an image inline, it means you have to click in the main window area after selecting an image from the thumbnail panel, or use the Edit->Copy image to clipboard menu item, which should always work.