Attachments are stored in the database by defualt, but cached on the file system. You might be hitting a choke point where it is taking a while to pull the attachments from the db (although I doubt it)--would you try downloading the same file twice, to see if that improves performance?
Regarding JDK or Tomcat settings, there's not much there to configure. You might want to check your system's memory and/or CPU to make that's not a bottleneck.
I would also try getting the image on the server itself to see if it is a server issue. If it's extremely fast, then you know that the performance is not on the server side, but is something related to the network.
I just got close to 300KB downloading a file from a Clearspace instance at work while sitting at home so I'm pretty sure the bottleneck isn't the servlet. Just to get some more information on the table:
a) Is your Clearspace instance fronted by Apache or IIS?
b) do you have a load balancer or firewall appliance in place anywhere?
c) what browsers are people using?