No, that is not possible.
In order to draw a colorbar you have to have a scalar value for each distinct color. You arrange the scalar values in order and draw the corresponding color at the correct relative position according to the scalar values, and call the result a colorbar.
However you can never do that for color jpeg. Jpeg supports scalar values at positions for grayscale (though this is very rare to encounter in practice), and jpeg supports RGB in which there are three values per pixel (not a scalar). Jpeg does not support indexed (colormapped) images, which is a system of linear values per pixel and an associated colormap indicating which color is associated with which pixel.
So color jpeg are never composed of scalar values, only of triplets per pixel. Therefore you cannot associate a scalar with each location as required to construct a colorbar.
