You can always use a cloud storage service like dropbox, onedrive, or google drive and synced the pictures on your mac and then on your iphone save them to your camera roll.
You can 1. create a folder on your Mac and sync it to your iPhone.
or 2. Create a folder in iPhone and sync to your iPhone.
I use the first option.
A. Create a New Folder on Mac - Label it. If you have multiple photos - create subfolders inside the folder.
For example - iOS Photos (street | candid | clients)
When you open your Photo App on your iPhone the (subfolders) will be placed under the last Folder on your iPhone. My last folder created on my iPhone is "Instagram"
B. Sync Photos Connect iPhone to Mac - open iTunes - Click on iphone icon - Click Photos - Select Sync button - Select folder