I use them often. I also do 2D animation as well as 3D so I found Model Sheets a important tool of both trades.

In Modo you use the backdrop Item and load the image to it. It is nothing but an image panel, most modeling apps have them called one thing or another. Using 2 crossed and lined up for front and side view. drop transparancy. Use them to model around.

Google Model sheets / what ever character or just model sheets and you will find lots.