You do not have the RIGHT to mess with someone else's computer. That includes the clipboard. It does not belong to you.
That person may have multiple windows open, where he is actively using the clipboard to move content between documents. He might even be doing this automatically under a script. If he pauses to check his email, and someone links him to your page, he is going to be really mad when his clipboard contents from the other process are disrupted. And if he gets someone's porn pictures in a customer's file (instead of his engine parts diagram), because you were messing with the clipboard, you might get slapped with a charge of computer invasion.
If your precious pictures are so valuable, then DON'T put them on a website. Suggestions on legal ways of doing what you want without breaking laws:
- If you are selling the photos, put low resolution versions on the site. Make sure you actually reduce the file size, not just the dimensions. Last week, I downloaded a thumbnail, and found the entire picture in the file - at 1200 px/in - I just had to enlarge it with a photo editor, reducing the resolution to 200 px/in in the process.
- If the user is paying for a download, I should think he should have the right to save a copy.
- If the person saved a copy for private use for research purposes, it is legal. The copyright law allows for scholarly research uses, and you have no right to prevent it.
- If the person downloads a picture for the sole purpose of preserving the ability to look at it after you remove it while changing your site, how is that hurting you?
- If you are worried that someone might use your pictures on other websites, post a warning that such use will be subject to litigation. If you find your picture on such a site, usually a notice of intent to sue will get it off there fast. I have already done that once, for an animation it took me hours to create.
- Realize that if the browser has downloaded your pictures to display them, they do not disappear from the Internet cache until he clears his cache or downloads something big enough to overwrite the memory. He can go to My Computer at any time and recover the picture from the cache. Also realize that you do not have the right to mess with his cache.
- Also, if he can see the URL of the picture (obtained in various ways), he can type in that URL and get the picture with no scripts running.
The Internet is designed for sharing information, not hiding it.