This tells the blind and visually impaired what you have on your page, while those who can see the graphic can enjoy the visual effect. That would make them invisible to non-graphical browsers. Your page would also appear much cleaner. The graphical browsers would still load the image and your page would look lovely to the sighted but be far easier to navigate for the blind and visually impaired. Please consider offering a text alternative link if you decide to use an image map on your site.
Image maps are absolutely inaccessible to the blind and visually-impaired.
If you are a graphic artist who offers graphics for download on your site, please consider the needs of the blind and visually impaired when setting up your site. By setting up your site so that it requires a click of the right nouse button, you are excluding the blind and visually impaired from access to your graphics. The image would appear to the sighted, yet the alt tag would be available to those who visit with their graphics disabled. Blind people cannot click on the right mouse button and download a file. Frames can be used on your site while still considering the needs of the visually impaired.