This is my first serious attempt at map-making and I have run into a major roadblock. How do I decide where to put mountains? I attempted making a basic height map (for reference only) by putting things in random places, but I am unsatisfied with the result. There's no reason to it and it looks unnatural. Without a height map, I am unable to put lakes and rivers in proper places.
This is the map that I'll use for the final product:
Click image for larger version. 

Name:	World v0_5.png 
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And this is my reference elevation map:
Click image for larger version. 

Name:	WorldElevation.png 
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*I only did the main continent, none of the islands are done

Other useful info:
--Using GIMP
--Land area is going to be 4,137,640 sq mi (a bit larger than Canada) on a planet that is a bit larger than Earth.
----Each pixel is 4 sq mi, 2 miles on each side

tl;dr version:
Where do I put mountains and other elevated land?