I've tested the EMail Settings API with many third party tools, and it seems to strip inlined embedded images (Base64), however the WebGUI can create inline embedded images in signatures with some browsers.
Has this been improved in GMAIL API Signatures? Will the new API accept (or continue to strip) inlined embedded images?
Gmail does not support inline image embedding for signatures, only image linking.
In general, the API doesn't provide any capabilities beyond what the UI already provides.
来源：https://stackoverflow.com/questions/38257996/new-gmail-api-signatures-settings