usergroup hack
 

I wonder is it possible when people register an account, they will first be in a usergroup which when they upload attachment, mods will need to approve them first. THen after a number of posts, they will move back to the registered group which mods won't need to approve their attachments?

Easier to set the registered usergroup up for that permission and then move them into a group which would allow them to do so, and take my word on it, this is the easiest way.

but then i need to move every single one of them manually?

No there is a mass usergroup move ;) or using promotions

but won't it be easirer if this can be done itself..?

Nope, much harder infact I know a few people, tried, failed, and took my advice.