But (at least for me) this did not work.
So I searched in the documentation but all seemed right.
The next logical step was "search on Google" (everyone should know that "he is your friend").
I read something on a forum regarding the function fltk::register_images() (Absolutely not documented) and I understood that it must be called before you use any other image-related stuff with fltk2.
So, if you want a widget to display your beautiful image you must code a main() like this:
You must then link the executable to libpng, libjpeg and libfltk2_images passing "-lpng -ljpeg -lfltk2_images" to the compiler.