......when I edit a copyrighted image and use it as a forum header?
Yes, if you do not have the author's permission to use the image.
OK, but how do you make your own header?
http://en.wikipedia.org/wiki/Image_editing
Will learn you the basics of image editing.
Yeah, but an image is needed first. The great majority of the images found on the internet is copyrighted. Does editing void the copyright?
What copyrighted image are you starting with?
editing voids copyrights, yes.
but there are lots of images with free licences, like public domain, or creative commons.
have a look for free stock image hosters, there are some which specialize in free for use images.
There are plenty of images without copyright online. Check http://commons.wikimedia.org/wiki/Main_Page
Or use the advanced search on flickr.com to look for images with an appropriate Creative Commons license (be sure to mention the photographer on your site as this part of the license). I often use images from these sources and I mostly find what I need.
I've searched for some free-to-use images but I have found nothing usable yet. I sent some e-mails to the copyright holders, I've got 1 permission. I'm going to make about 10 user-selectable color schemes with different header images, every image is copyrighted and hosted on different sites. :D
2 permissions.... :o ;D ;D ;D ;D ;D
Try http://www.deviantart.com too. Lots of artists there WANT you to use their stuff.
These are not real photographs but some edited stuff.