Thank you for the quick answer @Frank
I changed the sample_demo_x11/main.cpp source code so that
# define MAX_HEIGHT 480
# define MAX_WIDTH 848
After compilation, I tried it by running
./detect_demo_x11 -m 4 -p 848p.bmp
It does detect something, but the prediction is wrong (see below)
Do I need to change anything else in the code?
Thank you again for the help and a great end of the week 
Arthur