serving static content -- huge set of large images. Minimal features need, just as fast as possible.
nginx is the current favored choice. LigHTTPd still works fine, but is less actively developed nowadays. LiteSpeed is also a good choice, and may be the best if you want commercial support or a nicer GUI. All of these are very fast, raw speed will not be a meaningful competitive differentiator within this group of webservers.
dispatching dynamic content plugins -- think a web server that does on-the-fly watermarking or image transcoding. I'm looking for the fastest, lowest-overhead way of dispatching this.
Hmnn, a custom extension module to nginx is the lowest overhead option. But writing modules in C / C++ is seriously time-consuming. OP says "any language acceptable", well, if that is so then nginx with a C extension, or maybe Apache with a C extension to benefit from Apache's richer set of modules and documentation.
But realistically, who writes C code today for non-mass-market products? I would consider Python code with Tornado, or a similar event-driven webserver in a high-level language to be a better match.