Did you create the original HTML file using the Thunderbird Write new message window and then 'File' > Save as' > File and choose html file extension? Did you save the file to desktop or suitable location like ...My Documents/'Tbird Sig' folder? The image you include in signature - is that image also stored on computer in eg: 'My Documents/'Tbird Sig' folderfolder ? Confirm it always looks ok when you are creating a new email to send. Does it always look like a large image in a Reply OR only sometimes or only when replying to certain people ? When you 'Reply' - Is the content of the email you are replying to included in new reply email? Is your Reply (the text you write) above the content of reply email and is signature below your typed section OR appended at the very bottom - below everything and is there a -- separator just before the signature ? Images of what you expect and what you are getting would help.