Go to the photo gallery for the zoo you want to upload photos to and click on the "New Photo" button. You will be able to upload multiple photos at once.
It's worth checking through the photos already in the gallery to make sure you aren't uploading duplicates of photos already there.
Within the first 48 hours after posting, you should have an "Edit" button below the photo you can click on and delete the photo? (I can't remember whether that works or not).
Otherwise, just report the post and ask the moderators to delete the photo as a duplicate