Blocking ISP's isn't necessarily the best thing to do... and even if the whole of Africa was blocked via ISP's, some of the smarter hackers could learn how to get through that issue. Best you can do is to copyright your images with a watermark (a clear one across the page that can't easily be removed)
For example of what a watermark is, is more or less the logo or company name written on an image to make it harder for others to claim it as their own... especially so if it's through the entire image and not in the corner, bottom, etc... All photos on the
Lexicon of Parrotswebsite, explaining each parrot species, have been watermarked....
This site also goes to explain what a watermark is, and in short also how to make one....
And not only that, but to make it impossible to right click on the page...
Land of Vos has a great page that examples this! You can't right click without a box telling you the page has been copyrighted... of course there are less noticeable ways to copyright a page without telling you... but even when you right click nothing happens.
It's still possible to steal an image from a webpage, sadly... especially if you are quite savvy with the keyboard... thus in short no image is truely protected... even more so, if someone knows how to remove a watermark (it is possible, but more difficult if the watermark covers the subject of the image in just a way that by completely removing the watermark you end up with a strange appearing image.... such as the watermark, say a name, is slanted from top left corner to bottom right corner, and the subject is directly in the middle of the image.... or if you use a logo per-say, that covers practically the ENTIRE image...)