How come my image won't transfer to my watch?
Unfortunately, Fitbit's file transfer code is notoriously unreliable and slow. It can take several minutes for a slide to be transmitted, so you do need to be patient.
The process seems to be more reliable if the app is running on the watch before trying to send images. If the watch app hasn't received any images yet, it should say 'No Images' when it's ready to receive them.
If a file transfer is interruped part way through, there's a chance that the Slideshow app will be left in an invalid state. Uninstalling and reinstalling the app fixex this (sorry!).
Worse, sometimes the Fitbit app itself gets clogged with file transfers, requiring it to be restarted and/or having its data deleted.