I searched but it seems like the subject hasn't been treated.
Ok, I've read "Digital Signature Appearance" by Adobe, and find out how to put my own image when displaying digital signatures.
Looking at some tutorial from the iText authors, I saw that it is possible to specify different images to display on the signature validation
status.
This is a very cool feature, but unfortunately it is not documented at all in Adobe's documents... (I had to found a patent description document
to get more info)
Here is what I've discovered: the signature appearance is organized in several layers (4 defined by Adobe, the others by the document producer),
painted sequencially when showing the document.
What I'm looking for is the way to the various layers are used: one may be used when the signature validation status is unknown, another when
the validation was successful and a last when it failed.
For now, all my layers are painted, no matter of the signature validation.
(description in "Dynamic preview of electronic signature" is on page 9)