How large is your library? If it's just a few GBs, enable iCloud Photos and just see your photos automatically downloaded to your iPhone. If 5GB is not enough, 99c a month for 50GB iCloud is very inexpensive.
If you have hundreds of GBs of photos, use Flickr. They give you free 1TB storage for photos. The catch is uploading all your photos to Flickr might take a long time. The upside, free 1TB on the cloud. Hard to beat.