You use html code in the pin descriptions in Google Earth. This way the information you include with placemarks can be much more visually appealing and easier to read. You can also include pictures.
Inserting Images: You can insert images from your Hard Drive or from the web. In either case you must know their exact location, you can't "browse" for images on the hard drive. The html tag is as follows:
The quotes are important. If you don't have them the image won't show up. To find the location of an image from the web you just have to right-click on it and select copy location. You can then paste the location into the pin description in Google Earth. If you're using an image from the web the location is simply the url.
You can also use embed code found on the web. Many internet videos have an embed code that can be put into pin descriptions in Google Earth. Not all video sites will work inside of Google Earth.