would i face any legal issues if i take car pics from other sites with no watermarks and place my own on them, then upload them to my site ?
You can get sued for this, just because it doesn't have a watermark, it doesn't means the photo isn't copyrighted!
Celebrity pics can "often" be copied ...I am not a legal expert ....For can if your website is not popular - it is "ok" for you to copy. No one is going to sue you if you do not have money. But if your site gets popular - definitely you will have to place your own pics.
actually nvm.. i just saw a page where the owner says its okay to post them in other websites as long as you dont resize them
A client of mine did that and had to pay $10 000 in fines, he basically earn nothing from the image, just for his web site. Do be careful, take for royalty free collection such as scx.hu etc