I have no idea why this exists, I don't want to know.
Call randompug_init somewhere at the start, and then call randompug with your script as the first argument.
When the pug will be ready for you, that callback will be called, the first argument will be the sprite index.
If the sprite index is less than zero, then an error has occured.
If you did receive the sprite, don't forget to call sprite_delete when you don't need the pug anymore.
Also, call randompug_clear from time to time, when you're absolutely not expecting any pugs, to save memory.