Guest book security with C# and MS SQL

Question

I'm actually not sure where this question belongs.

I'm developing a guest book for my web site and am concerned about security.  I don't want a bunch of spam or nonsense written to my guest book by way of some automated nasty-ware.

Typically the security I see uses images of letters and numbers that the visitor must duplicate before access is granted.

The way I'm envisioning doing this is through an array of images that rotate on the web site and a parallel array of what the decryption is. Seems simple enough (although I haven't started coding yet, and we all know how perceived simplicity can change as a project is fleshed out).

Ok finally, here's my question.

Why are images used?

Using JavaScript I can easily render a random string of characters to the browser that won't show up in the html or be available through "View source" and authenticate it also using JavaScript (hmm I have to think a bit about that authentication..) before it is submitted. 

Has anyone out there tried this approach? All of my db backed apps have been behind firewalls with my company, so this is my first foray into the outside world and I want to do my best to be prepared.

TIA
Ken