What program are you using to create your heightmaps in? You need one that allows a 'Greyscale Mode', such as GIMP or Photoshop. Simply using grey colours won't work, you actually need this mode enabled when making the image.
If that's not the case and your map is in 'Greyscale' mode, check these things:
- 513x513 or 512x512 (image size)
- One layer
- Saved as a .png image
- Saved in your Feralheart >> Media >> Terrains >> Map Folder
- No alpha channel
- Named NAMETerrain.png
I would recommend looking at some of the map making tutorials, just to make sure you're following each step correctly. For example here's one by Crin using GIMP.
http://www.feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=17452.0Good luck