I don't see any reason why not to add alt text to the stock images on your website (I say this both as a site publisher and as a professional photographer).  The alt text doesn't imply ownership of the images, but rather it's designed to provide an assistive description as to what the image represents.  The only area where I think you would be deceptive is if your alt text claims something that is blatantly false, such as showing a stock image of a man and the alt text indicating that it's a photo of your CEO.

Descriptive alt text, including keywords, will have a positive effect on SEO.

I recently launched a new website.  During development, I'd enabled the option in WordPress to prevent search engines from indexing the site.

When the site went public (over 24 hours ago), I cleared that option. At that point, I added a specific robots.txt file that only disallowed a couple directories of files.  You can view the robots.txt at http://photogeardeals.com/robots.txt

Google (via Webmaster tools) is insisting that my robots.txt file contains a "Disallow: /" on line 2 and that it's preventing Google from indexing the site and preventing me from submitting a sitemap.  These errors are showing both in the sitemap section of Webmaster tools as well as the Blocked URLs section.

Bing's webmaster tools are  able to read the site and sitemap just fine.

Any idea why Google insists I'm disallowing everything even after telling it to re-fetch?

I don't see any reason why not to add alt text to the stock images on your website (I say this both as a site publisher and as a professional photographer).  The alt text doesn't imply ownership of the images, but rather it's designed to provide an assistive description as to what the image represents.  The only area where I think you would be deceptive is if your alt text claims something that is blatantly false, such as showing a stock image of a man and the alt text indicating that it's a photo of your CEO.

Descriptive alt text, including keywords, will have a positive effect on SEO.