These directories contain texture images. The .tif and .jpg files are the
original images in TIFF and JPEG formats. .bmp, .ppm, and .pnm images have been
sized to a power of two in each dimension and resaved. These textures are good
to go for OpenGL.
There are quite a few "swatch" types of textures in these directories. These are
images with several textures. To use one of those textures, use an image editor
such as GIMP or Photoshop to cut out the texture you want.