You can upload a Terragen file just fine. In order for you to understand what is happening, it is very important for you to understand that a Terragen file IS NOT AN IMAGE. A Terragen file is text. All Terragen files are text. What you see in your browser is the true contents of the file.
Terragen is a landscape program that lets you make pictures of scenery. When you save a .tgd file, you are not saving a picture of scenery. You are saving a series of commands to Terragen.
Think of it this way. Suppose I told you: "Take out a red pencil. Draw a circle one inch in diameter that is half an inch from the left and two inches from the top of the page. Take out a blue pencil. Fill the circle in blue."
What I just said is not a picture. It is text; my words are not a picture of a red circle filled in blue.
That is what a Terragen file is. It is a text file that contains a list of commands to the computer program Terragen. Terragen follows those commands, and draws a picture. But the TGA file is not a picture; it is a list of commands to follow in order to make a picture.
What do you want to do? Do you want to put up a picture you created in Terragen so that other people can look at it? If so, you need to "render" the picture--that is, you need to tell Terragen, "Open this list of commands, follow the commands, and then save the picture you made by following the commands." When you render a file, it will make a picture, which you can open in Photoshop or some other graphics program and save as a JPEG for placing on the Web.
