Use the Information | Digital Signature | Check Digital Signature command to verify that a particular signed picture is still the same as it was at the time of its signing. You can also perform this operation by clicking the "ribbon" digital signature icon shown on a picture's thumbnail.
Go to Settings | Preferences [Ctrl+M] and then the Signing Certificates section for options governing the verification of digital signatures. Verify that certificate has not been revoked: this option requires that you be connected to the Internet, so if you are not connected non-stop, it can be best to turn it off. The Only check revoking for signature certificates prevents the whole certificate hierarchy from being checked.