I don't think they're "animated" in the sense of cycling through sprites. I imagine their particle emission is currently hardcoded. (Didn't someone code a particle sheet at some stage? Are we using that code yet? Oh, wait, it's in the experimental TFTD branch.)
At some stage I'd definitely like to build a graphics data format, and just have the engine read preset data, so that people can mod a file somewhat like the ruleset to change what renders when, what displays particles, and how we choose various sprites to render, somewhat similar to the .nml format for OpenTTD.
edit: Was using the wrong file extension in my example. Whoops!