You can allow the client to set the caption under each listing image as you can see in the following screenshot:

When in the gallery optimized configuration only the larger emphasized image will have it's caption displayed. By default the software allows inclusion of the image caption. Also by default the software uses the actual file name of the uploaded image within the caption. You set the image caption within the image upload feature within the place a listing and listing edit process. Below is a screenshot of where to set that as well as what the software does by default within that field:

Within that screenshot you'll see the field to set the caption for each uploaded image as well as the image file name pulled by default configuration.

You have control of what appears by default or whether to allow the caption for individual images at all. You would look to the following settings in the admin tool:

LISTING SETUP > FILE UPLOAD & DISPLAY SETTINGS > MAX LENGTH OF PHOTO TITLE/CAPTION
LISTING SETUP > FILE UPLOAD & DISPLAY SETTINGS > STARTING TITLE

You can see those controls in the following screenshot:

Within that screenshot you'll see:

1. This is the max number of characters you wish to allow within an individual uploaded file's title/caption. If you do not wish to use this feature set it to 0
2. By default configuration the software inserts the uploaded files filename within the title/caption text box if title/captions are allowed. But with this setting you can set the be "blank" by default.