My guess is that GDI can keep with a moderate load, provided you're sensible
in the way in which you use your GDI objects.  For example, if you commonly
use images having identical dimensions, you might be able to reuse the image.
Similarly, if the output is often the same, you could cache the output image
in-memory.  Bear in mind that a webserver is a multi-threaded environment (it
serves requests in parallel), so any static/shared resources must be used in
a threadsafe fashion.

Good luck!