I had this problem and had to put the full path to the image to get it to work:
Image image1{Point{100,50}, "/home/user/stroustrup/chapter12/image_tester/simple_image.jpg"};
It would be really cool if the book addressed issues of path (relative/absolute or what?) in fltk. But for now I'm just using the full dang filepath because that's what works! In the book it just has "image.jpg" or whatever but that was giving me the same error as you no matter what I did.