1. An image forming apparatus comprising: four first imaging units that use standard toners for colors of yellow, magenta, cyan, and black, respectively; a second imaging unit that uses a toner having a particle size smaller than a smallest one of particle sizes of the four standard toners; and a transfer unit that transfers first images formed by the respective first imaging units and a second image formed by the second imaging unit to a recording medium such that the first images are provided on the recording medium and the second image is provided on the first images.