The only problem here is that there are newbies whom wanted to posts images while cannot wait for their activity to reach junior member. As long as you post with 30min and up interval for atleast 10 times a day, you can be easily reaching the Jr. Member rank for a week, and also if you are a good poster and helpful to be worth merited.
Maybe we make requirement more strict to significantly reduce abuse possibility, such as :
1. Must earned at least 3 merit and have at least 15 activity (which means they must active at least for 2 weeks)
2. Must earned at least 6 merit
Even so, only very few newbie (who're not spammer) who can enjoy the benefit.
Keep in mind that theymos did mention this:
Limiting newbie participation is very harmful for a community. Newbie jail will never return: I consider the newbie-jail period to have been extremely damaging to the forum. When barriers to participation are too high, then the best people often just won't go to the trouble of joining, and the people who are willing to jump through the hoops are often people who aren't good for the community: people with nothing better to do, scammers, get-rick-quickers, etc. Having a permanent newbie jail policy would improve things a lot in the short-term, but would end up being a fatal poison to the community.
Though you are suggesting some perks and beneficial to the newbies that are good posters, it would simply be a hard task for them with such ceiling of standards.