With the cube selected, go to edit mode.
Click on face select mode.
Right click to select one cube face.
Open a new UV image panel.
Put your mouse cursor over the 3D view panel and Hit U with the face selected.
In the UV image panel click open image
In the 3D panel, hit plus to open the 3D panel's properties menu.
In the 3D panel view, go to the properties menu - shading - textured solid (this will turn materials off and turn textures on).
Hurray! Now you can see your textures!!
However, if you try to render the image right now the cube will just be rendered gray :(
Also, your materials will not visible in the viewport (unless you turn textured solid off). If you want render your image with the textures there are some more steps that we need to take:
With the face selected, go to the texture menu.
You can try adding a texture to the monkey's eyes too!
Here are some textures that you can use: