Can the rescue external HD be the same as the HD used to store my images?

Possibly.  First you'd want to make sure that your system supports booting from USB "fixed disk class" devices like HDDs and SSDs, as opposed to "removable storage class" devices like flash drives.  Not all systems do, and if yours doesn't, then obviously that won't be a practical setup for you.

If that isn't an issue, the next thing you need to look at is whether your external HDD is initialized as MBR or GPT.  If it's GPT, then a) you'll only be able to boot it in UEFI mode, so hopefully that's how your system is set up to boot, and b) Rescue Media Builder currently doesn't support building to targets initialized as GPT, so you'll have to do some light manual work.  Basically, just create a small FAT32 partition (shrink an existing partition if you don't have any unallocated space), then have Rescue Media Builder generate an ISO file.  Mount that ISO by double-clicking it, then go to the virtual DVD drive you'll see under This PC, and copy everything from inside that ISO file onto that FAT32 partition.  When you're done, you can right-click the virtual DVD drive that was used for mounting your ISO and click Eject.

One additional note: If you're planning to keep this external HDD connected all the time, then you might still want to have separate Rescue Media or at least a Rescue Media ISO somewhere else.  The reason is that if your system ever gets hit with malware that trashes your external HDD, that could wipe out your Rescue Media partition, which would make it more difficult for you to recover from one of your Reflect backups.  The purpose of Rescue Media is obviously to rescue you when things go sideways, so I would advise having a Rescue Media build of some kind stored somewhere that is not persistently online and subject to attack