Ehh x-x Try explaining better please. ..I've got lost..but if i understand right your misstake is that you've name your map Terrain.png" and it have to be : UnnamedTerrain.png(example)
If you explain your problem better i might help more
Okay, so from what I understand something is wrong with your height map. I suggest opening Watermill's Height map and pasting in yours with watermill's settings.
Make sure it follows everything below
one more time: [Quote Source: Map Maker Manual] If you were to create a heightmap on your own there's a few preferences it must follow and it has the very same as the heightmap used above and below. When your create/save a heightmap it must/should follow these rules.
It must be saved into a PNG format
It can only consist of 1 layer!(If you use more the game will crash upon importing it) Make sure you flatten/merge the layers on the image before saving it.
The image must be saved/edited in grayscale mode and cannot consist of or use colours(If not the game will crash)
The Image size should be even on both size, the standard size is 513 x 513 pixels.
The use of "Alpha Channel" should be left unmarked/turned off.
I suggest you read this: Map Making Guide in Gimp (http://feral-heart.com/index.php?option=com_content&view=article&id=6&Itemid=11)
Good luck!