Generally, using Canvas2D (and a CanvasTexture) with multiple images is like this:
clear the 2D canvas
draw the images on the 2D canvas with desired position, scale, rotation
set the needsUpdate to true of the texture (this will automatically update the 3D model)
Whenever you change something (e.g. adding a new image, scaling existing image, etc), you repeat the 3 steps above. Yesterday I already posted some source code. Did you see it: