This isn't really a Project-GC issue.
If you're the cache owner or the person that posted the photo you can visit the log and delete the photo.
If it's your own log you click the "View / Edit Log / Images" in the bottom right corner of your log on the website, then you click "edit image" and then "delete image".
If you're the owner of the cache you click the "View log" in the same place, then click "edit image" and then "delete image".