Signed Signature Field

I not sure if I'm missing something.
The documents we work have signature fields. The annotation fields are created by other tool(iText, I guess).
When the android apk attach the signature file (a bitmap generated by a Wacom activity) we can see it right away on the pdf.
But when I open the pdf document in the Acrobat Reader, this happens (see attachment).
Do I need to do something when I apply the signature bitmap to the annotation? Or is this something not related to the radaee api?
I know there's a method on the Annotation class called SignField, but I not sure if that's what I need and how to use it.
Any ideas?

not sure what operation on the pdf file, did you change appearence on signed field?
please do not change appearence when field was signed, this cause the digest data not match data changed before.
we has special version, which bigger than normal version, can sign the field with a cert file, in that method, you can customer appearence of field.

if you mean Acrobat draw a focus rectangle on a signed field, just after you click on the field.
i shall tell you, this behaviour is standard action of Acrobat.