Why? I just like old race cars and collecting pictures of them. What is the limit for?
There's currently a limit of 2000 downloads per logged user per day, there might have been a lower limit when you wrote your message though. Basically one can download about 1 wallpaper every 30 seconds for 16 hours straight before hitting the limit. I think that's pretty permissive actually.
The reason there's even a limit for this is because sending all those big files down the internet, and reading them from the disk, is pretty expensive, and one person abusing the download feature could slowdown the website for everybody else.