Images will need to be followed from their source through digitisation, addition of metadata, archiving, editing and delivery. As an example, if the images require editing or further manipulation as a result of quality assurance checks, you will need to implement tracking procedures to provide access and version control. Likewise, you may wish to retrieve images that have been digitised using a particular capture device, if there later proves to be a problem with that piece of equipment. In search and retrieval, there is a trade-off between recall and precision. Recall is the number of items retrieved from the total number available that would have been relevant.
Precision is the number of images that are relevant against the number that are irrelevant.
In a search function, high recall is balanced against low precision, and high precision is a trade-off against low recall. The effectiveness of a search and retrieval function is measured by precision. The role of serendipity and iterated, exploratory searching in which requirements evolve through interactions with the search system complicates any attempt to define an objective measure of relevancy. There are always trade-offs involved when judging the appropriate level of detail needed in your image metadata. The TASI metadata paper discusses in more detail the issues surrounding the nature of the textual metadata associated with an image.