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While, as @Jeff said, you can't encrypt your HTML output, you can obfuscated it to make it more difficult for a human to understand. Here is a link to a free product that does just that.
See: http://farhadi.ir/works/rc4
Two simple libraries allow you to encode in PHP and decode in Javascript. In your output simply set up the HTML skeleton and then run the decrypt with the key and encrypted string in a document.ready function and populate the <body> tag with the product of decryption.
Of course, this only works with Javascript enabled. Otherwise the use won't see anything useful. You're also passing your secret key in the clear which is fine since you're only trying to prevent "novice" users from accessing your source code. I also don't think it will really help you out that much in preventing someone from grabbing an image from your site. You're much better off working with .htaccess to do that. Check out http://bignosebird.com/apache/a13.shtml or Google
You could be ultimately cheeky and use this: http://www.dynamicdrive.com/dynamicindex9/encrypter.htm
It basically encrypted your HTML code into a javascript document.write line.
Not really encryption, but will deter over half the people browsing the source.
Enjoi.