How to transfer a photo from my laptop (mac) to my iphone?

How to transfer a photo (PNG) from my laptop to my iphone.
I've read in another post that there's a photos icon in my itunes but i don't seem to find/have it.

To transfer a photo to your iphone you must first open iTunes and then select your iPhone. Once you have done this you will see a bar across the top of your iTunes just under the music player which will have options on it like "Summary" "Apps" "Music" "Movies" and further across to the right there will be the option "Photos" Click on this and select the folder where your picture is in so that next time you sync your device the picture will sync with it.

Here are ways to add items from your computer to your iTunes library:
1. Drag a file or folder from the desktop to the iTunes window.
2. If you add a folder, all the files it contains are added to your library.
3.In iTunes, choose File > "Add to Library," locate a file or folder, and then click Choose.

You could email it to yourself.
If you use dropbox you could put it into your public folder and then grab it on your phone.

Or you could just turn on "sync photos" for the phone in itunes, but specify just a single, empty folder. It will default to your pictures folder but you don't want everything copied over. So make an empty folder in your pictures folder called "iphone," put the one photo inside, and sync just that folder.