I hate to say it, but you answer your own question. "I noticed that you can't post copyrighted stuff..." If you see an image on another website, or in a magazine / comic / newspaper, the automatic assumption is that those images are copyrighted to the owner of that medium. Outside of images that are clearly marked as 'public domain' or 'creative commons' as mentioned above, you would have to contact the appropriate person and ask their permission.
I faced a similar situation with my website. I wanted to use a very particular type of image for my index page to give an immediate impression of what the site is about. Being the terrible photographer that I am, I knew that I wouldn't be able to make the image myself, so I did a Google Image search for the type of picture that I wanted and then had to find one that was free to use. Speaking of, if you go to images.google.com and click on advanced options, there is an option to search only for images that are believed to be under the creative commons license, however even they will admit that it's not perfect and you have to verify the image's status for yourself before you use it.