Hi Joerg
You should probably take a look at an emitter, and set the “Shape” to “Geometry” and use an image or video in the shape of the dragon for the “Shape source”
The existing particle emitters in the Pyro section will make a good starting point for experimenting.
For instance try to take the Corona emitter, change the shape to geometry an drop a picture (with alpha channel) of a dragon in the shape source well.