ॐ शुक्लांबरधरं विष्णुं शशिवर्णं चतुर्भुजं |
प्रसन्नवदनं ध्यायेत् सर्वविघ्नोपशान्तये ||
Om shuklambaradharam vishnum shashivarnam chaturbhujam|
Prasannavadanam dhyaayet sarvavighnopashaantaye ||
This verse is generally chanted as a prayer, to invoke the Grace of Lord Ganesha, before undertaking any work. The Lord is invoked as Vighnahara (remover of obstacles). One meditates on Him for the cessation of obstacles. He is shukla-ambara-dhara, the one who wears white clothes. He is vishnu, all pervasive and shashi-varna, of the color of the moon. That is, he is not scorching but pleasant to think about. He is chaturbhuja, has four hands and is prasanna vadana, a smiling and tranquil face.
The form of the Lord is described for the purpose of visualisation. We do not worship the form but the lord for whom the form is given.