Description
Annotation refers to associating descriptive text with an image, commonly known as metadata. At its simplest this could be a single unstructured text field associated with the asset, but this simple implementation won't support the variety of purposes that users may expect from the annotation feature. Here are a sampling of annotation purposes. This is not an exhaustive list, but rather an attempt to show the variety of possible annotation types:
Keywords or tags
Alt text
This annotation supports accessibility. Alt-text is descriptive text that can be shown if the image can't be displayed due to browser incompatibilities. It can also be read aloud for access by the sight impaired.
Caption
A caption is a one line description of the image, such as would appear near the image to identify it.
Description
A more substantial body of descriptive text.
Source
The image source describes details of the image's origin; who created it, and whether it is copied from another site, or is an original.
license for use
Images posted on web sites have either implicit or explicit licenses for use. While many sites specify no license for the use of their content, it is prudent to know how copied images are licensed, and to include a license for original work.
Related links
Rating
Metadata schemes
Annotations incur another technical challenge. There are a variety of standard schema for annotations. A common one is Dublin Core, another is VRA. Each of these schema define the name and purpose of annotations so that an application can correctly interpret their content.