Mapping images to faces on a mesh requires the use of both the "3D View" and "UV/Image Editor" Blender windows. Before beginning the process, be sure that you have both of these window types visible on the screen at the same time:

If you're not sure how to arrange blender windows or change their types, see Working with Blender "windows".
- With the 3D view in "Edit Mode", select the faces that you wish to map to an image (It's also a good idea to set the 3D view's selection mode to "Faces").
- Press the 'u' key to unwrap the selected faces using one of the available options on the "UV Unwrap" menu that appears.
- Load the image that you wish to use in the UV/Image Editor window by selecting "Open..." from the "Image" menu, if you have not already done so.
- Use the image selector pulldown in the UV/Image Editor window to assign the desired image to the selected faces:

See also: