In most of my games I use images and sounds/music I find on the net.
That is a HUGE no-no, unless you're specifically getting them from legit paid download service (eg a sound effects or image library that explicitly gives you permission to use the SFX or images in games). If you just "google" and grab images or sounds, there's an excellent chance that you are in violation of copyright of the sound, music or images.
To be clear: EVERY image, sound, etc is copyrighted. And unless you specifically have a license to distribute it, you're technically in violation of the copyright. Now, the rights holder may not mind, but that's a pretty big gamble to take.
Always, always get images, sounds, music from reputable sources; there are many places online to get them, usually for a nominal fee; sometimes free.
There is only one way. That is to obtain a license. (yes, there is such a thing as 'fair use' exemptions, but not if you're making a game based on someone elses characters..)
Generally, what's the best way to avoid copyright infringement if you want to create famous games-related projects?
So as not to rain completely on your parade, it may be worth pointing out a story... The creators of "Red vs Blue" (the HALO-based machinima) were clearly in violation of Microsoft's copyright-- they used the HALO characters/art and put their own voices onto it. (technically speaking, they created an "unauthorized derivative work"). However, rather than slap them with a lawsuit, Microsoft thought it was really cool--and ended up giving their blessing and helping them. So you never know what'll happen....
Brian Schmidt Studios
Executive Director, GameSoundCon