Hi Thong,
If you are using the latest version of the PDFNet Android SDK, this is done for you already.
When using the signature tool, check "Make Default Signature", then, the next time when using the signature tool, you will get an option "Use Saved Signature" which will insert the image either on to the page, or into a signature field.
The source code can be found in the PDFViewCtrlTools project src/pdftron/PDF/Tools/Signature.java.
If you have any questions, please don't hesitate to reply again.
Best Regards,
Shirley