Appearances are optional for at least the Author signature, though the approval ones it is expected they are added.
There is a new API to allow you set the appearance from an image in the Annots.SignatureWidget class.
SignatureWidhet.CreateSignatureAppearance(Image)
https://www.pdftron.com/api/PDFTronSDK/dotnet/pdftron.PDF.Annots.SignatureWidget.html#pdftron_PDF_Annots_SignatureWidget_CreateSignatureAppearance_pdftron_PDF_Image_
Example usage here.
As for the image itself there are a variety of ways to create one, for instance creating a PDF page and using PDFDraw to convert to an image.
Or if you want to add one manually you see this post.