@hempomatic
Leaving aside the Apple ID business just for the moment. Deleting 1500 images from your iPhone is....well...a nightmare and will exhaust your goodwill towards others as well as driving you mad
So, don't try to do it manually. This works:
Attach your iPhone to your Mac. Once connected, open Image Capture app found in Applications. You should see your iPhone listed on the left. Click on it.
Now, Click on the first (or any) picture. With that picture highlighted, use Command plus A which will highlight ALL your pics.
Now, Right Click on any pic and from the options, choose Delete, then Delete All. This will delete all your pics/videos etc. It will take time, but you can clear off and do whatever you want. I once had to delete 19 thousand pics/videos. That took about 6 hours if I recall correctly; so 1500 shouldn't take too long.
NB. You have to take into consideration what Jake wisely said about "iCloud". He gave you two options: deleting from iPhone AND iCloud or just from your iPhone (which requires that you turn OFF iCloud Sync).
Ian