There is no limitation on the number of tags that can belong to a particular image. The annotation capabilities are extensive. For example, Sticky Notes attach to the image with an icon. Embedded Notes appear to be part of the image. In a single search phrase using multiple words, both sticky and embedded notes can be searched.
Redactions block out portions of the image and can be labeled for identification.
Markups can be drawn on the image for attracting attention. Voice and Video Clips can also be attached and played with readily available video and voice software. The test of a good retrieval package is its ability to provide you with meaningful information - not just images. The numbering system that is most familiar to you can easily be implemented. Legal professionals can quickly cull through documents from their desktops and electronically organize the information.