You just didn't connect the dots. The technique that sites like Tripod and Geocities use to prevent "their" images from showing up on "foreign" pages depends on the browser supplying the URL to the page on which the image is being displayed when it requests the image. When you type the URL in the address bar directly, there's no enclosing page and the browser sends nothing. Thus, the server can't tell whether the usage is "legit".

Bit o'trivia: The URL of the enclosing page is called the "REFERER" field in the request. Note the misspelling--it's enshrined in the standard, and can't ever be changed without breaking millions of Web servers and hundreds of millions of browsers. Standards live forever...for better, and often for worse.

--Robert