To add keywords to accepted images you first click on the word "Dashboard" on the top black menu bar of your contributor page.
Then click on the image you want to make changes to.
Then hoover over the right side within the text box beside the key word to view a pencil icon.
Click on it. A list of your existing keywords will open.
Scroll to the end to add the words you wish to. Fifty (50) is the maximum allowed.
To make changes to caption, On the same page you make changes to you keywords hoover over the right side of the text box with the caption, you'll see a pencil icon. Click on it. That will allow you to make changes within that text box.
I hope this was helpful. Please feel free to ask any further questions.