It looks like using bordeaux-threads is the *only* option that works for me :(

I use bordeaux-threads: make-thread to make a thread, download the picture in that thread, and after it's downloaded, emit a signal. I think the slot function is still executed in the main thread, not the new thread created by bordeaux-threads:make-thread, am I right? At least I didn't observe anything suspicious. Correct me if this is not a recommended way.