Different browsers are able to and configured to display different types of documents themselves. Browsers are usually configured to handle other file types by using helper applications or by offering to save the file. There is no way for a web author to override the way the browser is configured to handle any given type of file. You can't, because the Web doesn't work that way. There is no way for a web author to override the way the browser is configured to handle any given type of file.
Most browsers allow the user to download to disk if they want to.
A thumbnail image is just a copy of the full-sized image that has been modified to reduce the size of the file. Thumbnail images can use multiple techniques simultaneously. Since many browsers display anchors with colored underscores by default, they will show the spaces to the left and right of the image with colored underscores. There are two basic approaches. The most cache-friendly method is to use a normal IMG tag that refers to a CGI script that randomly redirects the browser to one of several images.