One of the most confusing elements of buying and selling stock photos is understanding how they can be used. Images are subject to copyright, which places limitations on how end users are permitted to use them. When buying a stock image from a microstock or stock photo agency, you're actually only buying a license to the image. This license dictates the usage rights that come along with the photo, which is typically limited to your own personal use.
Personal use of a stock photo generally means using it for basic, non-commercial purposes. This includes featuring a photo within a blog post, e-book or other content. In these cases, the photo is intended to accompany existing content without being the commercial basis for the content itself. Most stock photo agencies do not permit license buyers to resell the image in any way, as the photo's creator or owner still holds the primary copyright.
Commercial use of a stock image includes reselling the photo by using it in someone else's work. This is a common offense by web designers, who often use stock images they've purchased when designing clients' websites. This would also apply to ghostwriters who create content for a price; by including stock images they've purchased, they're effectively breaching their personal-use license to the photos.
Derivatives includes selling items prominently bearing the stock photo, without the copyright holder's permission. For example, taking a stock image and printing it on mugs, mouse pads or T-shirts would be considered a derivative use of the photo. This is not included in most stock photo agencies' standard licenses, but extended licenses can often be purchased to support derivative or commercial use.
In short, you typically can't resell your stock photos. Without proper licensing or express permission from the copyright holder, reselling a stock image for commercial purposes may be grounds for legal action. This could lead to extreme measures such as being forced to cease and desist with using a prominent image, or having to deliver all monies earned via the image to its copyright holder. For these and other reasons, it's best to read the license's fine print before using a stock photo.