You can do this
image = sf::Image();
But the question is: why do you want to unload the image but keep the corresponding instance?
If it is cleared in the destructor, why not load the image in the constructor?
SFML doesn't use exceptions, therefore it never does anything that may fail in a constructor.
Moreover, images can't always be loaded at the moment they are constructed, so a default constructor + a Load function is always necessary.