Qualifio offers you different possibilities to request any information. The caption, for example, is not a standard field, but it can be added when creating the contest.
In step Questions of your upload campaign, you can define any open fields that your participants have to fill in – optionally or mandatory:
By default, this information is visible to all participants in the zoom preview only. The caption can also appear in the gallery (thumbnails) if you check Display the participant’s answer to this field in the gallery:
When this option is checked, the caption will be displayed when hovering over the photo like so: