You cannot texture just one side of a DB primitive. As you can see from Mojo10's second snippet it textures all six sides.
To do what you want with a texture, you would have to do the texturing in a modelling program first.
If you want to be able to do it at run-time, your best bet would be to create a plane (DB calls them plains) the same dimensions as one face of the cube and glue it onto the cube.
You could then texture just the plane and it would look like the cube face was textured.
Here's Mojo10's second snippet ammended to do what you are after:
A$ = " SIDE 1 "
text 0,0,A$
get image 1,0,0,Text Width(A$),16
make object cube 1,40
Color Object 1,RGB(255,0,0)
Make Object Plain 2,39,39
Set Object Transparency 2,1
Glue Object To Limb 2,1,0
Offset Limb 2,0,0,0,21
texture object 2,1
autocam off
position camera 0,40,-200
do
YRotate Object 1,WrapValue(Object Angle Y(1)+.1)
loop
TDK_Man