Is this an accurate depiction of the internal shape of a turbofan engine? This image (so commonly used here that the first image reference for it that pops up in a Google search for "jet engine diagram" points back to ASE!)

From Wikipedia

Shows the housing of the actual "jet" section very tightly packaged around the various sized compressor and turbine stages, leaving a diverging/converging path for the bypass airflow. 

It seems to me that it would make more sense for the packaging of the inner jet engine to be a fairly even cylinder so the bypass air flows smoothly around it and out the back of the nacelle as I've attempted to depict with the blue line at the bottom, or a somewhat converging shape as I've depicted with the red line at the top.

Source: personal modification of above mentioned Wiki picture. Don't laugh... I drew that with my thumb-operated trackball.

However, as I've typed this question up and looked at the diagram more, I can see where the converging path around the low-pressure turbine section of the jet would add additional pressure to the bypass air and, therefore, additional energy to the escaping air.

Is this simply an "artist's concept" of a cross-section of a turbofan engine, or is it an accurate depiction of the internals?

Note: I do understand that within the jet section of the engine the shaping needs to be tightly packaged to ensure the jet-internal air flows exactly where it's needed, my question is about the flow of the bypass air only.
Strictly speaking, no. It is an artist's concept to help us understand the engine schematics better. A better figure would look some thing like this :



GE-90 Cutaway; image from web.stanford.edu

The blue colored region more or less shows the 'cold flow' region (where the bypass air flows). This graphic is better for two reasons- not only does it show that much of the flow takes place in the cold flow half-ducts, but it also shows that how the pipes and tubes that are all over the place are covered.

The figure you linked in the question doesn't show the cold air flow ducts, for simplification. Also, the shape and size of the bypass duct varies from design to design (though they are similar overall).



Differences between two and three shaft engines; image from 1.bp.blogspot.com

In the above figure, you can see that the though both the similarly shaped bypass ducts converge, their location and length is different. Of course, in the real aircraft, these ducts host the thrust reversers, as explained by @mins elsewhere.