When you have core “image” field on your node type and you want to output “title” (or “alt”) of this image just under the image thumbnail as text - you can use image_caption_formatter module for that.
But only in the case when you don’t need another image formatter in place - e.g. Colorbox - because Colorbox provides its own image formatter, and image_caption_formatter module also provides its own image formatter. You obviously can’t use two formatters at the same time.

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