I backed up all of my photos to iCloud from my iPhone 6 but it didn't leave any thumbnails on my phone it left the originals. So the photos are taking up significant space in iCloud AND on my phone and I need space on my phone. Help!

You need to do the following:

Can I use iCloud Photo Library to save space on my device?

iCloud Photo Library automatically keeps all your photos and videos in the original, high-resolution version. Follow these steps to choose how you store your photos and videos on your device:

• On your iPhone, iPad, or iPod touch, tap Settings > iCloud > Photos or Settings > Photos & Camera, then select a storage setting.
• On your Mac, click Photos > Preferences > iCloud, then select a storage setting.

If you turn on Optimize [device] Storage, iCloud Photo Library will automatically manage the size of your library on your device, so you can make the most of your device's storage and access more photos than ever. All of your original, full-resolution photos and videos are stored in iCloud while device-size versions are kept on your device. You can download the original photos and videos over Wi-Fi or cellular when you need them.

If you turn on Download Originals, iCloud Photo Library will keep your original, full-resolution photos and videos in iCloud and on your device. Download Originals is the default setting for iOS devices with the free 5 GB storage plan and for all Mac devices.

Here are a couple of iCloud Photo Library support articles that may be helpful also:

iCloud Photo Library FAQ - Apple Support

iCloud Photo Library Help - Apple Support

Cheers,

GB

If it is saying your iCloud memory is low, then you need to buy more iCloud space than you currently have. The photos in iCloud are full resolution. The ones on your device are optimized, and the iCloud Library function will manage the number of photos you have on your device. It will only store the ones in optimized format that can fit in the available memory on the device. Those photos that you do not look at often will be put into thumbnail format as space requires.

Cheers,

GB