How to preventing image browsing throw an application in c#?

I have a website to show some pictures to my visitors.

Pictures URL is something like this:

http://ift.tt/1RLSO1P

Now i was thinking it's possible to write an application to generate image URLs like

http://ift.tt/1RLSO1P
http://ift.tt/1MCs73Q
http://ift.tt/1RLSO1R

to brows all pictures throw an application.

Some one can write an application and steal all pictures. Pictures have been watermarked but how can i preventing to this type of fake browsing?