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.