How much information can one send in a bitcoin transaction?

I'm not a programmer, but I love the idea of using peer to peer networks for payment of goods and services. Especially if the service or goods are digital downloads (movies, apps, music, games, ect.) Now I've heard of the ability to include information, such as contracts and ID information within a bitcoin transaction. My question is, how much information can someone send? Can they include pictures, mp3, video?

As a musician, I wonder if a media player merged with a bitcoin wallet could solve intellectual property rights issues. For example, if an open source program that is a bitcoin wallet and plays media thru unscrambling parts of the block chain, can copyright owners have their royalties paid directly just thru bitcoin transactions? With all the news about ASCAP and BMI having issues with pandora and spotify it would be a great innovation.