John Carter:
"How easy is it to convert both C++ programs and C++ programmers to D?"
Porting C code to D is not hard, it's mostly mechanical work, you just have to keep an eye on few things (like passing fixed-sized arrays to functions by reference, global floating point data not initialized to zero, etc).
I think that for a C++ programmer it's not too much hard to learn D. But converting C++ code to D could be very hard (and a slow work) if the code uses lot of small things that are specific of C++ and missing or different in D. If the C++ code is more plain, then it's probably not hard, despite the missing multiple inheritance and struct inheritance.
Converting CImg to D seems a multi-years work. Bye, bearophile