Question: I have many JPEG files (pictures) on my USB flash memory. I give it to my customers to look at those pictures and choose one (then I make their order based on that selection). 
I want they can look at the pictures but don't can to copy or drag-and-drop them. 
Is there such a way?

I use Windows 7.

Answer: 
Unfortunately, if the file can be read - its raw data (1's and 0's) can be copied my merely reading them and putting the same bits into another file.

Even if you figured out how to copy-protect a memory stick, there are other things which users can also do including screenshots, print-n-scan, drive cloning, advanced file unlockers.. the list is endless.

Unfortunately, because of this - it is impossible to stop someone from copying the images.

You could always embed the images intoa piece of flash or similar - making copying the files directly harder to do. Couple that with a watermark and they have something not worth copying and requiring a it more effort - which will stop the basic copyist, but you can never stop everyone.
In short, no. If they can look at the files, they can copy them. There is no way in Windows 7 to prevent this.