@ Stinger30AU The plain text mode under Edit > Preferences > Mail preferences > HTML Messages is set to "show HTML if present". Format message in HTML is checked under Edit > Preferences > Composer Preferences > General. And last but not least all my signatures are in HTML format. So that all seems to be as it should be to make an external image appear in a signature..
P.S. I just had an Evolution error. I wonder if the signature issue and the error are related:
[97587.696393] evolution: segfault at 4 ip b6bc4f50 sp bfba1230 error 4 in libgobject-2.0.so.0.2000.1[b6bb8000+3c000]