There's at least two methods to use reference images in your workspace.

1. By using the Maya's built in image plane. Go here and take a look at the free video tutorial:
https://www.gnomononline.com/modeling.html
Some people are having trouble using the image planes as they might slow down Maya's performance.

2. Another method is to use nurbs planes positioned and scaled correctly. Then you create shaders with the reference images in their color channels and apply them to the appropriate planes. After that you create a layer and assign the planes to it and change it's mode to R (reference), so you don't accidentally select or move the planes. If I recall correcty, the monster tutorial on the VIP area shows this method better than I can explain