Sometimes you can get this error message from Gmail if you paste in your HTML signature from Microsoft Word or Google Docs. WHat happens is that you may have unknowingly pasted in the styling from the external program.
You can remove the styling using the “Remove formatting” option from the editor’s tool options. But this will turn your signature to a plain text signature with no styling and no images at all.
So what you need to do is use an HTML editor from the web to remove the extra styling but preserve the rest.
You can also save yourself the hassle, and make a professional email signature using the free email signature generator (it takes just 2-5 minutes).
Generate your signature