Background image as a fill texture

Images can be in any of these formats; use powers of 2.

Material

Set up the material to flat color white ... (flat color material will not receive shadows)

Texture image

import a jpeg or other image ...

crop and fill document ...

 

Transparent model

(To see the reference image thru the model)

Method A -- One layer with Fast Shader and RGB Intensity
This method is efficient because it is in one layer than with two.

Select the object that you wish to sculpt.

Select the Fast Shader
(this material renders faster than any material with the Transparency modifier)

Set Rgb Intensity to between 50 and 80.

This must be done before the object is drawn.

Draw the object.

If you like the amount of transparency, press T to enter Edit Mode.

If you want to change the transparency, press W for the gyro and slide RGB Intensity.

Hit T to enter Edit Mode.

Method B -- Two Layers with a transparent setting on a material

Start out with a material that already has transparency. I used Jelly Bean.

Set the Material/Modifiers/Transparent slider by increasing transparency as shown:

 

Turn off flatten

Render/Flatten off

(Note --Turning Flatten Layers off in the RENDER panel allows you to have transparency in a layer.)

Draw/"RGB Intensity" to under 70.

Pick a material with a transparency setting (like jellybean.) Then set Material/Transparency to 100.

Make a second layer. Draw on the second layer