Also, the upload goes to /images/dev/[mac addr] until the image is completely uploaded and moved to it’s final home at the end of the imaging processes. The chances of the upload process leaving you imageless is small, but the chance your new image is worthless due to some configuration issue, is medium.
If you have a large number of devices, you can create a new image definition and upload it just to make sure it works, then run a simple SQL statement to point all devices that used the old image ID to the new image ID.