I wonder whether there is any risk in placing a scan of one's diploma online.
The diploma itself isn't supposed to be a secure document, and is given to people to post on walls, not to present as verification of a degree. A quick google image search reveals that many, many people have posted copies of their diploma's on the internet, so it's not like this would be a rare commodity. Degree verification is done directly through the college/university, not through diplomas.
As long as you're comfortable revealing personal details of your life to everyone, I see no reason not to post a diploma. Keep in mind that posting personal details about yourself makes you a bit more vulnerable to Con-men who can try to convince you to trust them based on this. i.e. casually mentioning they went to the same school and got the same degree as you just a few years before/after you did.
Not really. Someone could of course copy & paste it, edit the fields, whatever. You have the original and are thus able to prove that you have it. I know some personal websites where people have posted their certs and diplomas and even though I wouldn't do it myself, there are no security or privacy flaws here.