How many posts do I have to make to get to jr. Member? I'm assuming activity and posts are not 1:1
post count doesnt affect activity that much, only if you have potential activity. to get activity you must be active atleast every activity period so you can get 14 points every 2 weeks
The reason I ask is because I'd like to make a new topic for a service with images. I hope I don't have to wait too long to do so. Are you saying I'm going to have to wait a month to become a jr. member?

2 weeks and four days to be precise (because this activity period is almost over).
of course you still have to reach at least 30 posts to become a jr member.