I received several great suggestions when I asked for feedback on using the GIMP with thin clients. Some were complex, some were expensive, some involved a switch from Windows to Linux since Linux seems to do a better job of managing memory. Others pointed to different thin client technologies (besides Windows Terminal Services) like NX and even VNC.
Ultimately, in the interest of time, money, and effort, I followed one user's particular bit of advice:
Identify the problem, then work to mitigate:
Lets face it any graphic manipulation program is going to consume plenty of resources.
Have you identified what resources are strained (disk, network i/o, memory/swap, processor)?
Once identified you can fix (move class to their own disk, add network card, increase RAM, relocate swap, increase processors or run gimp at lower priority).
So a closer examination of the processes and memory involved made it very clear that the problem was RAM (or the lack thereof). Again, as other users pointed out, this is an area where Linux can shine by handling application footprints a bit differently. Fortunately, though, I was able to avoid a complete system migration and just re-allocate labs to different servers.
Previously, we had 3 25-seat labs (plus a scattering of thin clients around the building) accessing 2 terminal servers. The GIMP was running in one lab and shared a server with a classroom lab running productivity and Web applications. The second terminal server was dedicated to another classroom lab and the other clients in the building. We have been looking at ways to add additional computer lab space, so this second server would have covered the new lab. By simply moving both classroom labs to a single server and allowing the second server to handle only the lab from which students were editing photos, performance improved drastically.
The take home lesson? Reasonably powerful terminal servers (2GB RAM, dual-core Opterons, etc.) have a practical capacity of 50-60 clients if they are all running basic productivity applications. Without looking beyond Terminal Services and the hardware we had, 20-25 clients running the GIMP was far more reasonable for the single server.