Use an image in MyDrive in your signature
We can do this by including a link to a drive image with Public Link Sharing enabled and replacing "file/d/" with "uc?export=download&id=" and removing "/view?usp=sharing" from the end of the shared link. This is only possible if your G Suite Domain policy is set to share outside the organisation.
For example this link copied from Google Drive:
Use Ctrl+H in Google Docs to quickly find and replace "file/d/" with "uc?export=download&id="
This link is then inserted into the Source field in the signature editor under Insert > Insert/edit image.