Go to the Add/Edit tab, select the desired listing and click Manage Photos.
Double click the photo you want to caption (or click the little icon in the top right of the photo slot):
Click Enter Description:
- Enter the caption.
- Click Done.
- Click Save.
That will bring you back to Manage Photos and allow you to add captions to other photos if needed.
Once you are all done, remember to click Certify and Save at the bottom of Manage Photos to save all your changes.
If you do not click Save at the bottom of Manage Photos, your changes/captions will not be saved.