How can one repent from unintentionally taking pictures at a wedding, and from the presence of personal wedding photos in an abandoned house that someone might have seen?

If a part of your 'awrah becomes visible to someone who is forbidden from seeing it due to your negligence or oversight, you incur sin proportionate to your negligence. However, if you have taken preventive measures, the sin falls upon the one who looked or photographed. You should request the deletion of the picture, and if they do not comply, there is no sin upon you. It is also advisable to destroy pictures in your home for fear that someone who is not permitted to see them might do so.