To add metadata to an image in Photoshop go to File > File info
It's pretty straight forward. Fill out the fields and hit Ok.
One thing to remember is to fill out the Copyright information in the description field. Why you might wonder? This really is important because websites like Facebook use the Document title and Description as their photo caption.
You can save the File information as a template and use it for other pictures. Click on the template button at the bottom of the window and then choose export. Don't forget to name your template and save it.
Open a new picture and have a look at the file info.
As you can see, the file information is empty. Instead of filling out all the fields I can just use the saved template to fill it. Click on the Template button at the bottom of the screen and choose Import. Browse to the saved metadata template and choose Open.
Now this screen appears.
Hit Ok and your metadata is now in the file
You can make some changes, like for example the title for this specific picture, and hit Ok to save the metadata to your image.
Shortcut for the File Info = Shift + CMD + OPT + i (Shift + Ctrl+ Alt+ i on windows).