In response to your proposals
I don't really see how your first proposal will prevent a child from being exposed to an inappropriate content posted by spammers.
As for your second proposal - it could well work, but you'll need to employ 24/7 moderation or much of the content will be locked, which is not very useful from users point of view.
I think you might be approaching this challenge from the wrong angle (content-locking rather content-moderation).
Text can be inappropriate as well
You should also very well consider that even if no images are attached, textual content can be highly inappropriate to underaged visitors; let alone that spam marketing is something no visitor would like to come across.
A bomb waiting to explode
If you have ever run phpbb or wordpress site, these are bombarded with spamming attempts, and I've even seen sites that had to shut down for a couple of days just to allow the removal of thousands of spam posts.
A site that requires no registration and has no challenge–response authentication for posting is essentially a bomb waiting to explode.
Moderation strategies
The real question is who moderates the content (how and how frequently), where you have two options:
- Site moderators - where you have someone constantly monitoring posts.
- Visitors - where visitors flag inappropriate content that is then being reviewed by moderators.
The former system requires frequent moderator presence and could be extremely time-consuming for moderators. But if the site hasn't got many visitors it could be the only practical choice.
If this is the path to be taken, you may wish to consider creating a joint moderator email address and give the access details to all your moderators. Then either instantly or periodically send an email to that address with a link to all the new images/posts. This will allow moderators to review the images on mobile devices and without having to be logged into the admin system. If all is good, the email can be deleted so other moderators won't have to deal with it.
The latter system is more appropriate for sites with many visitors, say more than 50 at any given time.
Stack uses such system in the form of a 'flag as spam' option, which takes super-high priority on moderators queue. I think it is reasonable to say that any spam will be removed within less than 40 seconds on stack sites, but there are many visitors and moderetors online at any given time.
Youtube has the same 'flag as inappropriate' option, which I can only assume gets dealt with on a next-to-instant basis.
Google employs the same strategy for its image search.
Summary
So I wouldn't stick to any of your proposals. Instead I would focus on:
- Providing a flaggin option for visitors.
- Providing an easy, quick and mobile-based way for moderators to moderate content.
