Is it okay to use generic stock photos for a blog?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  David Kutcher: Negative is that 1) your lead images on social sharing links will be generic, and 2) you won`t benefit from organic image search.
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  Jeff Jockisch: With generic stock photos, not only do you miss out on most of the SEO value of organic image search but your ability to build trust is hindered. Unique and original imagery that relates to and enhances the understanding of the words on the page is often a key element in time on page and a differentiator in conversion rates.It is possible to highly modify a stock image into something that becomes original - but of course then it’s really no longer ‘stock.’
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  Nathan Davidson: Yes it is
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  Michael Martinez: It is okay to use generic stock photos for a blog. People use them all the time. John Mueller of Google confirmed they don`t penalize or downgrade or downscore sites. But if you`re hoping to be found in image search then you should use unique, distinctive images as much as possible. Regardless of whether your images are unique, be sure your ALT= text describes what is in the image (i.e., don`t just put targeted keywords in there) and use captions to enhance the value of the images.