texture not visible

I have created a quad that I want to texture with some image I have. But somehow it stays invisible or (depending on the image I use) gives me a strange color (might very well be one color used in the image.
So somehow I think I haven't quite layed out my texture properly. Could anyone of you tell me what I am doing wrong here?

Graphics3D 800,600

SetBuffer BackBuffer()

camera=CreateCamera()
CameraViewport camera,0,0,800,600

;setup lighting
Light=CreateLight(1)
RotateEntity Light,45,20,0
PositionEntity Light,640,300,640
LightColor Light,160,160,100

cube=CreateCube()
PositionEntity cube,1,1,6
tex0=LoadTexture("tree.bmp",1)
entitytexture cube,tex0


sprite=CreateMySprite()
PositionEntity sprite,0,0,5


While Not KeyHit(1)

pointentity sprite,camera
TurnEntity cube ,0.1,0.2,-0.3
UpdateWorld
RenderWorld

Text 340,500,"Texturing Demo"

Flip

Wend
End



Function CreateMySprite()

tx=Loadtexture("wall.bmp",1)

mesh=CreateMesh()
surf=CreateSurface( mesh)

AddVertex surf,-1,1,0
AddVertex surf,1,1,0
AddVertex surf,1,-1,0
AddVertex surf,-1,-1,0

AddTriangle surf,2,1,0
AddTriangle surf,3,2,0

scaletexture tx,2,2
entitytexture mesh,tx

UpdateNormals mesh
Return mesh
End Function

By the way, the cube does get textured well...

Looks like you're missing UV coordinates.

Change

AddVertex surf,-1,1,0
AddVertex surf,1,1,0
AddVertex surf,1,-1,0
AddVertex surf,-1,-1,0

to

AddVertex surf,-1,1,0,0,0
AddVertex surf,1,1,0,1,0
AddVertex surf,1,-1,0,1,1
AddVertex surf,-1,-1,0,0,1

ok,
that worked!

silly me.
Thanks!