hi you know what, i was wondering if this is possible

i have a website and it looks something like this:

http://www.example.com/?wallpaper_id=456

as you can see from the above url, anyone can view all my pages because its abvious that my url increments to anyone can view my wallpapers and can download with robots all they have to do is increment like this

http://www.example.com/?wallpaper_id=451
http://www.example.com/?wallpaper_id=452
http://www.example.com/?wallpaper_id=453
http://www.example.com/?wallpaper_id=454
http://www.example.com/?wallpaper_id=455
http://www.example.com/?wallpaper_id=456
http://www.example.com/?wallpaper_id=457
http://www.example.com/?wallpaper_id=458

and so on.. this is the way im getting some robots to crawl my wesbite and sometimes they just put my content on their site

so i want to have my urls like this for example:

http://www.example.com/?wallpaper_id=A32dxhsjdW

as you can see from the above, its only a code kinda like youtube if you see youtube they have have their urls like that

can you help me im not very familiar with encrypting or encoding my code.