1) Yes you can create wehsites with the own "/images" directory.
This is illustrated in tutorial video CP-05
www.jms2win.com/en/tutorial#jms12x-CP-05
2) Yes you can use a single DB.
In this case, under joomla 1.5, we recommend to never use the "jos_" table prefix for the master website in case where you would use the default (jos_) prefix when creating a slave site.
Under Joomla 2.5, this recommendation is not required as Joomla generate a random table prefix.
3) In general, yes it is recommended to have the symbolic link available to have a specific "/images" directory because almost no extension are reading the configuration of the "media manager" that specify the images path.
We recommend using the Symbolic Links. So if they are available, this is an excellent news.