In addition to the vital element of technical excellence addressed previously, I suggest that you consider each image from the position of the stock house. Stock images, you can assume, are not sold for display in the home, but rather for business use in ads, catalogs, and other commercial applications.
The subject matter should be generic enough to be considered for appearance across a broad spectrum of products or services so that the stock house could anticipate multiple sales. Still, it should be different enough to separate it from the countless other images of the same subject. With a common object or setting, and marketability in mind for example, have you presented a unique vantage point and/or lighting? Unusual choice of lens focal length? Adaptability to vertical and/or horizontal cropping? These and other considerations, in addition to technical excellence, weigh heavily on the potential resale of each image, and of your success in this crowded market.
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