Which database should be used when developing a client-server application as a retail product?

We are in the process of developing a client-server software (in C#, Net 2.0)which can be sold off the shelf. The product is in Medical domain.

Can anyone suggest the best possible database for the following conditions:

1. Should be able to distribute it free to the customers.
2. Customers should be able to add images too to the database.
3. Small footprint.
4. Fast, stable, secure and does not crash.

We are working on MSSQL 2005 Express and Mysql 5.0. Any better database which you can suggest?

A good idea would be to write your software using NHibernate, as the framework supports many different databases with only a change in configuration settings. The reason I suggest this is because clients MAY have a database license already and they may want your software to use that database type.

So I suggest keeping your app flexible while offering an off the shelf database recommendation.