You can load up to three reference pictures at a time into Hexagon. Each picture will correspond to a separate plane's axis (x, y, or z). You can then use these pictures as a guide to your modeling project.
To do this, go to the Scene Tab. You can make sure it is turned on by going to WINDOW --> Scene. Choose the second tab, 'Properties,' and scroll down. You will see three fields for each plane of reference. Click on the '...' button next to the field to browse and load a picture file (.jpg, .bmp, .png, .gif, or .tif).