Another tip would be to use Image plane or Spotlight to overlay your reference image. The information would prove valuable for proportions and gesture.
The diffuse map is the image you've already done. It's the base color image. It has a bunch of other names, but diffuse is technically the most correct.
Lightbox is a file loader/ image browser. Spotlight allows for several things like image manipulation and projection-painting, but it's still using polypaint.