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Here is how I do it. This keeps all my photos I want synced in a single folder. I created a folder and named it iPhone. All images I want to sync with my phone are in that folder. You then sync them to your phone with iTunes. Very easy to do.
Plug your phone into your computer and open iTunes.
Go to your phone in iTunes.
Next go to photos tab. This will give you a menu that will allow you options to sync the photos to your phone. As you can see in my screenshot.
I use this folder all the time for syncing images between my phone and iMac. I have done it this way as I create wallpapers and need an easy way to drop a wallpaper onto my phone for testing fit. And then easily delete it from my phone after I have tested it. Hope this helps you.