|Whatever I've decided to throw at you...|
NEVER UPLOAD FULL RESOLUTION FILES ANYWHERE
If your art still gets stolen or reposted to another site with out crediting you, ask the person to credit you or remove the art from their site. If they refuse, feel no guilt in reporting them.
This will not erase ALL art theft, but it will certainly help to prevent plagiarism. If someone is intent on ripping you off, nothing will stop them. However, as an artist you certainly can make their job a lot harder by only posting low resolution and watermarked images. Again, try to include your website URL in your watermark and it will not matter as much if your image is reposted somewhere with out crediting you.
The following is a tutorial I found quite useful in creating a simple watermark in Photoshop or Gimp: