Captcha Alternative, how secure?

You need to make it difficult for an OCR system to read the text. Otherwise the spam bot will easily get through your captcha, without any customisation from a human spammer.

That's why you see funny XORing, noise and distortion on most captchas these days.

As a matter of principle, it makes sense to NOT base the image name on something that can be looked up, although in the case of a local library, chances are low that any spammers will be writing custom scripts to defeat your captcha...