You need 2 Windows.
1. 3d view.
2. Uv/image editor.
-3d view.
Delete all (a/a x)
Num 1
Add Cube/tab.
Add plane/s scale bigger.
Num 3 / move plane in +y direction.
Tab.
Num 1
Add Camera.
Num 3
Move Cam in -y direction.
Num 1
Select Cube.
Tab / u /unwrap (smart projection).
UV/Image Editor.
Open an image (image/new/select one).
-3d view.
Tab.
Select plane (RMB)
Tab / u /unwrap (smart projection).
UV/Image Editor.
Select same image as for the Cube before.
-3d view.
Tab.
Select Camera (RMB)
Strg/Control +num 0
F9/Camera panel switch todo orthografic.
Select Cube (RMB)
Add modifier UVProject/write in the field ob: Camera (or however your Cam name is).
Select plane (RMB)
Add modifier UVProject/write in the field ob: Camera (or however your Cam name is).
Select Cube (RMB) / Alt+z.
Switch todo Texture Paint mode.
Add a new UV texture layer in Mesh panel.
Tab.
UV/Image Editor.
Image/new/ok.
-3d view.
Tab.
Paint panel: activate clone/ +clone layer.
Opacity 1,0
Size 66,5 *hrhr*.
Paint over the Cube/ move the Cube and Paint again over it (with g and LMB).