I had a BMP file which I wanted to transpose and turn into raw RGB 5-8-5 values. Using the Python Image Library made things absurdly easy.
That's Python's advantage in a nutshell. Things are just so absurdly easy.
If there was an equally good C++ image library available then it would have been equally simple in C++.
Of course the thing is that, in general, there may well NOT have been such a C++ library, and this is what makes languages such as Java and Python popular - because of the breadth of libraries available.
For C++ image manipulation I've found libvips to be decent, but the point still stands that for a given problem there may not be decent libraries available.