First, use drawing software to open the signed picture to be inserted.

Edit the text signature of the message in a new message,

In the "View" mode of the picture viewing software, click the right mouse button to "Copy Image".

Click the right mouse button to "paste" to the place where the picture needs to be inserted in the new email signature.

Copy the edited signature (including the picture) into a new email.

Open Tools-Options-Mail Format-Signature, then paste and click OK.

Try to create a new email. The picture is the original size.