Now let's create some mountains!
Load up "MountainsLayer.png" into GIMP.
Again, create a white layer at the bottom to make your efforts easily visible.
To the "Background" layer, apply Layer->Colors->Hute-Saturation with 0/50/0
Duplicate the "Background Layer", then apply a Gaussian Blur with 5 px to the dublicate.
Then create another dublicate, and merge that with the blurred image.
Repeat this step four more times.
Label the resulting layer "Blurred" and make the "Background" layer invisible.
Duplicate the "Blurred" Layer.
Apply "Script-Fu->Shadow->Drop-Shadow" with 5/5/15/Black/80% to "Blurred".
Apply "Filters->Artistic->Oilfy" with 3 px to "Blurred".
Reduce the Transparency of the layer to 50%.
Apply "Gaussian Blur" with 50 px to "Blurred Copy".
Apply "Filters->Artistic->Oilfy" with 10 px to "Blurred".
Merge the "Blurred copy", "Blurred", and "Drop-Shadow" layers.
Now the mountains look good. However, if you want to combine the mountains with other terrain features - such as vegetation - problems will soon become apparent. You can test this by filling the while layer at the bottom with a different color.
To be ready for such future changes, create a new, transparent layer called "Foundations".
Go to the combined "Blurred" layer, and apply Layer->Transparency->Alpha to Selection.
Go to the "Foundations" layer and fill the entire selection with white five times.
Now make the white background layer invisible and save the rest as "MountainsColor.png".
And now you can combine the result with the saved "OceansColor.png". You should place the "OceansColor.png" layer on top of the other layers to avoid any terrain features growing into the ocean!