QUOTE
now that [I know] what a Terragen file is, now I understand that I cannot put it in that form into the webhost
But, of course you can. It will simply need to be used by someone with Terragen. If the .tgd file is smaller than the jpg you've been using, you can save some space ( and up/download time ) by using the Terragen file. And, since this is supposedly a Terragen forum, everyone should already know how to open/view a .tgd.
At least, the image would be at full resolution instead of whatever your jpg gets reduced to.