I believe these instructions for Outlook Express should apply to WinMail.

Note: HTML Signatures must be less than 4KB.

Open a New Message window and create your signature. (Insert graphic &
text). Click the Source Tab. (View | Source Edit if not already checked).
Highlight the entire source code and copy / paste into Notepad. File | Save
As and in the File Name field, save as an htm file, (i.e. Sig1.htm). It will
be saved to My Documents by default. Go to Tools | Options | Signatures.
Click: File | Browse and Insert your saved signature | Apply. You can rename
it now if you wish.

Of course, if the recipient is reading in Plain Text, they won't see it.