can Photos app be installed on an external drive to use for backups ?

(I would just download same pics from camera twice). I now have High Sierra and want to backup my Photos Library on an external drive.

You can still do that - just double click on the library you want to use to open it - but be aware that is easy to get confused and mess things up by using the last used library instead of the library you want to use

Can I just install Photos app on an external drive to use as backup t

No - the Photos application must always be in the applications folder and there is no need to ever back it up as it is part of the OS and trivial to recover by reinstalling the OS

Can Time Machine be set to backup just one app (Photos)?

it could but that again would be pointless as the App is unimportant - the Photos library is critical that is where you photos and all other data are stored

I used it years ago and remember it backed up EVERYTHING that was done on the computer,

By default Time Machine does backup your entire computer - and if you are not backing everything up then you are 100% guaranteed to lose things sooner or later - computers, disks, people and software all fail and if you choose not to have complete current backups then you lose thing software they to - by default (and the very best plan) Time Machine does hourly backups so when thing fail you lose at the most a hour's work

If you choose to lose other data you can backup only your Photos library by dragging it to a properly formatted drive (Where is it safe to store a Photos Library? Requirements for the external drive) or by using Time Machine excluding everything except the Photos library or using other software like SuperDuper, carbon Copy Cloner or others

LN

Back up the Photos Library, either manually or with a product such as Time Machine; to access it, choose Home from the Finder's Go menu and double-click the Pictures folder.

The Photos application is part of the OS and is backed up with it instead of individually.

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. I don't care if the backup app has editing features or anything thats going to hog space - just want the photos backed up. I'm not a fan of "the cloud", however.

Just a note on iCloud, since mentioned it. You would use iCloud Photos Library, if you have several Apple devices and want to keep the Photos Library identical and synced across all your devices. But it is no backup. You cold also use iCloud to save storage on your devices. But even if you use iCloud to store your Photos Library you would need a backup to keep your photos safe. I am keeping one backup of my photos at home and one offsite, at my place of work.

You are welcome - when you have more questions come back and ask

LN