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Bit tricky inpure 2d as there are no alpha commands.
If you have B3D out might want to use a sprite.

Graphics3D 640,480
cam=CreateCamera()
MoveEntity cam,0,0,-1
sp = LoadSprite("grass1.png")
SpriteViewMode sp,4
alpha#=1.0
While Not KeyDown(1)
EntityAlpha sp,alpha
alpha=alpha-0.005
If alpha<0 Exit
RenderWorld:Flip

Wend
End

(might be a better way of doing it as I don't use sprites often). There's also a 'setgamma' method somewhere.
Alternative is using extended B2D DLL.
extended b2d dll
or writepixelfast...
writepixelfast
writepixelfast2
or Skidracer's fade...
fade