For tiling, just set the texture coordinates to be greater than the size of the image. (If I remember correctly, this only works 'properly' with power of 2 image sizes, otherwise you get borders in between tiles.) You can even have negative texture coordinates.
e.g. to tile an image across the screen

CONSTANT Width = 800, Height = 600
STARTPOLY 123, 2
POLYVECTOR 0, 0, 0, 0
POLYVECTOR Width, 0, Width, 0
POLYVECTOR 0, Height, 0, Height
POLYVECTOR Width, Height, Width, Height
ENDPOLY