Your bump map should have the same dimentions as the color image.
In the Materials Buttons, on the far right side, is the Texture panel, containing a vertical list of buttons. Your current image map should occupy the top.
Click the second and ‘add new’. Press F6 for the Texture Buttons. Make the material type ‘Image’ and load the bump from the panel that appears (just like you did for the color map image).
Now F5 back to the Material Buttons. Click the ‘Map Input’ tab of the Texture panel and make it UV (just as you did before for the image map).
Now click the ‘Map To’ tab. Un-click the ‘Col’ (color) button. Click ‘Nor’ and maybe also ‘Spec’.
Hope this helps.