Thank you all guys! Really helpful
You’re absolutely right guys! What should i do when somebody copied my work thought?
I know that there are some lines already on the preview image but are not strong enough to protect the work, for example somebody with basic skills can remove them and take theft the logo. The resolution he gets would be low ,but he can use it in websites at least, not for prints. So i am talking about adding something big as this in deviant art, look at how my logo is there with the watermark: http://fc00.deviantart.net/fs71/i/2013/011/9/8/info_talk_logo_template_by_blinvarfi-d5mmhey.png there is no way somebody can copy that!
I have a suggestion. How about envato create a watermark for files preview, at least for logo marketplace. A watermark like those used on DeviantART (i think everybody knows those). It would be a good way to prevent coping of our logos don’t you think?