Q: What happens to Photos back up to iCloud if you erase an iPhone?

singingangel wrote:

If I use Settings > General > Reset > Erase All Content and Settings on my old iPhone, will it erase photos on my new iPhone or in iCloud or just make them disappear from the old iPhone?

No they won't erase as they are present on the new phone - in fact if you are planning on selling or giving away the old phone that is a recommended action to be taken

What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support

However it is also a good idea to get those photos to a computer, external hard drive or another off device storage like Google Photos or Upthere.com

Import photos and videos from your iPhone, iPad, or iPod touch to your computer - Apple Support

To be on the safe side I would import the photos from your new iPhone to your computer before erasing the old one. The imported photos are far more secured than an iTunes backup. Here is how. Follow the steps for Mac or PC.

Import photos and videos from your iPhone, iPad, or iPod touch - Apple Support