is it safe to post art on 'net

You could scan your drawings onto a computer and then use a free photo manipulation program like GIMP to add a watermark.

 

Generally speaking, it's difficult to prevent anyone from using anything you post online. What's suggested above is really the best approach; add a signature or watermark of sorts. If you paint digitally (Photoshop, Gimp, Corel etc) it's pretty easy to add your own mark on a different layer. If your work is done through natural mediums, you could always add a signature on the paper (or on a different paper in which you overlay when scanning it, if you don't want the signature to ruin your art).

You can also try googling "add watermark online", through which you can find many easy watermark generators. You just upload your image and choose a watermark and it does the job for you.

If you're using Photoshop or want to, I'd be happy to give some quick pointers -- though there are already countless tutorials online