Where did you get stuck when converting? Note that
sf::Image has been split into
sf::Image and
sf::Texture.
Apart from that, the code of your tutorial is quite questionable, maybe you should rewrite your own or take parts of it
#define ABS(a) ((a>0)?(a):0)
ColorScale::const_iterator start = std::map<double,sf::Color>::begin();
ColorScale::const_iterator last = std::map<double,sf::Color>::end();
class ColorScale : protected std::map<double,sf::Color>