Thumb nails take up less space on the current page to allow for other things and so the page loads faster. They actually take up more room on the server because it has to house the pic and the thumbnail.
As far as the 10 post rule for pics, I think it's worthless, because it does not allow newbie to get thier info to the people with the answers. As far as spammers and bots they will just progam the bot to make 20 posts in 2 second and they are in, so that will not stop them.
The not allowing PM's until 10 post I think is good, except PM to admin if a newbie has tech issue, I had that issue on another site where I put in the wrong email at sign up and did not get my password and could not get anyone to return my emails so I had to create a new account.