All very good advice so far. I really don't have much to add. Except that maybe I should do a backup and restore video to explain it hands on how to do it.
I concur, stay away from Dropbox. Their Files integration is absolutely horrible. If you really want to stick with Dropbox, use their app to upload / download files. You can use the Share dialogue to send files to Dropbox or Share files in the Dropbox app with Stage Traxx.
Regarding pictures: They will not interfere with ST3 but are absolutely useless. Just delete them (after creating a backup).
The file with the ST3 icon from your last screenshot, that's the backup file. And this is the only file you need to store safely or transfer to another device.