Are you attempting to create a signature within Thunderbird 'Write' window which you intend to save as a HTML file and then attach to emails? This would be the best way to resolve the problem. https://support.mozilla.org/en-US/kb/signatures section 'Signatures stored in files' which also has a section following regarding images. You can control position of text and image by using a table and placing text and image in cells. 'Insert' > 'Image' and locate image method and not copy paste.