Stop Showing Signature Images in the Inspector Sidebar
By default, Postbox shows inline images in the Inspector sidebar. This has the side effect of showing images contained in HTML signatures in the sidebar as well.
If you prefer to not see signature icons in the sidebar, you can turn this capability off by setting a hidden pref.
Open the config editor (see: How to Open the Config Editor)
Type "mailnews.display.overrideDontShowAsAttachment" into the "Filter:" textbox and double click on the that row for mailnews.display.overrideDontShowAsAttachment to change the value from true to false.