In my opinion a simpler and more effective way to avoid the problem would be to try to improve the communication with those users, at least a bit.

You see, it seems that the main case that's generating the problem here is when: The user uploads a picture that doesn't get approved within a couple of days; and The user doesn't know enough about the site's rules to know why that is the case

With that situation in mind I'd have a couple of suggestions on how to reach out for the users and try to educate them on why that may be the case:

• Tweak the "Create new forum" page to detect if the user has pending approvals and add a message on the top in a similar way to the "you have dmail" one, preferably saying something to the effect of "You seem have posts that still need to be reviewed by the staff, please do not create a topic about it. Instead here's a helpful link to our uploading guidelines", bonus style points if we create a sticky with the title "My post hasn't been approved yet. Help!" to link to, which would contain all the info about how mods and etc look over all posts and only really skip them in some cases (all info that I don't remember being in a single help topic but rather only through word of mouth? I might be mistaken though)
• Tweak the post page in a similar way, as to display a message "This post hasn't been approved yet. If it's been several days be sure to check our forum post about how posts get accepted [URL]". I suggest this change as I feel that the post itself might be one of the places the user would most often keep checking again and again before they finally decide to make a forum complaining about it. Same deal with the post index page, with a message like "Some posts in this page have not been approved yet. If it's been a couple of days make sure to check our forum post about how images are accepted [URL]"

No need to introduce complex systems that would not be used by the majority of the staff approving new posts. A couple of helpful signs on places where people often get lost are much cheaper than a permanent rescue team dedicated to getting them back on track all the time.

Just an idea though, I'm 100% sure people could implement it much more elegantly than this