Merge textures in GIMP
In this video I explain two different ways to merge textures using layer masks in gimp. At the end of this tutorial you'll be able to add snow to the ground, sand, ashes...
Try to play with the settings I use in the course of the video to achieve the effect you want. For example, get the black and white cursors of the colour levels settings closer to achieve more contrast, use a different colour than black for the shadows etc.