The marketing team where I work is looking for an application that would allow their team to categorize and sort a slew of images and have them be read/write for the whole team. My understanding of the request is that they're looking for iPhoto or Adobe Bridge on a commercial level. The database and images would need to be accessible from multiple people, on multiple machines; preferably stored on a Windows file share. The number of people using the application could range from 5-50 people depending on how widely the solution gets adopted.
Often the first step in finding a solution is to put a name to it and in this case it sounds very much like you are after a Digital Asset Management package.
There are many of these available; iView Media Pro used to be held as a benchmark and this has had a few transitions of ownership and is now sold as Media Pro 1 by Phase One. Another option could be the Portfolio Suite of products from Extensis.
There are many more each with their own strengths and you need to review these along with the marketing team to find the best fit for their workflow.