After you do the update you no longer need the image file anymore.
It may take some fiddling to find a USB stick that works without a hitch for firmware updates. Assuming that USB worked well, maybe consider reserving it (the USB stick that worked before) for only firmware updates. Then next time you can use the same stick and the update will work well next time.
You can always archive the firmware ZIP files on your computer although there is no real need. If you're a hoarder it may be comforting to archive these. But you'll probably never touch those again.
Hi Tom - Jason got that spot on. I keep a 2Cb USB2 stick solely for updates, formatting it each time a new update comes out. It's done the last 3 faultlessly. It is not necessary to keep the old ones, as each new one incorporates all the old ones. In any case it is important that ONLY the new update is on the USB so that Montage can find it easily. All the update downloads are stored in my computer 'Downloads' folder anyway, should I ever want to revert ... which seems unlikely, although I do wish they hadn't made the Motif Pfs part of the Pre-sets ...