Go to Settings, then into your account which is the first option. Go to iCloud, then Photos, then select 'Download and keep originals'. Do that and wait for a while depending on how many images you have.

iCloud is pretty weird. If you have the 'Optimise phone storage' option enabled, it uses iCloud to store the original quality larger files but saves lower quality smaller files on your device. So, if you choose to download the originals, iCloud has to delete all the smaller files and replace them with the originals. It might take some time. If you're not in a hurry, just select the 'Download and keep originals' option, then wait a few days before disabling Photos in iCloud settings. I think the whole process only happens if you're plugged in too, but I could be wrong.