This video explains how to use an image file (eg. a JPEG) as a texture for an object in Maya. You can use an image to texture an entire shape or plane, or just selected faces of a shape. Watch the video to find out how. You can also download printed instructions from here.
Note: To apply a texture to just selected faces of a shape rather than the entire shape, right-click the shape and click on Face to enter Face Mode. You should then be able to select individual faces and right-click to assign a material to selected faces. This allows you to use different textures for different sides of a shape.