Yes u can import Your Pics into a *.wad on WED !!
Open WED then Texture Manager->New Wad-> SAVE!!!
Now in the Scrollbar u find *.wad right Click into the Window and Add Texture!!
Mark ure Picture and import
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Tutorials:
[Blender]Terrain creation ENG/GER
[Blender]Low Poly Tree Modeling
[GIMP]Create a Texture for Terrains
CLICK HERE