A photo says it is “still being prepared” — is it lost?

No. The photo is saved; only its smaller versions are still being made, and it appears once they are ready or once you publish it.

No, it is saved. What is outstanding is the preparation: stripping the camera metadata, baking in the rotation and making the smaller versions the gallery grid uses. Until those exist there is no thumbnail to show, so the queue shows a label instead.

This is normally seconds. If it has been much longer, reload — the queue does not poll for you.

A photo whose preparation genuinely failed stays out of the public gallery rather than appearing broken. That is the safe outcome by design: nothing is published before it has been prepared and checked.