This problem is caused by the color model of the image. Images used for color printing are usually in a color mode called CMYK, which refers to the four inks used in color printing (cyan, magenta, yellow and key/black). This color model is different from additive color models, e.g. RGB, which are being used by computer monitors. The biggest difference between these color models is that when combining all colors of CMYK together the resulting color is black, while for additive color models all colors combined result in white.
Images using the CMYK color mode are poorly supported on computer monitors, and, depending on the monitor and webbrowser, may result in negatives of the image.