Paste the link in to a Google Doc or Notepad. Use CTRL+H to find "file/d/" and replace with "uc?export=download&id=" and remove "/view?usp=sharing" from the end of the shared link. (This is only possible if your Google Workspace domain policy is set to share outside the organisation).
For example, this link copied from Google Drive:
Now, copy the new edited link from Google Doc / Notepad.
On the Email signature creation page, place the cursor where you would like the image to go. Then, select 'Insert' > 'Image' and paste the new link into the Source field. There is no need to enter a Description or edit the Dimensions.