Captioning an image in Keynote
The Description field attached to an image is not designed to be visible in your presentation. Instead, the purpose of the Description field is to provide additional information that can be read by assistive software. This allows users of any such software to access your presentation in a meaningful way.

Assistive software is primarily used by people who face challenges (e.g. visual impairment, learning difficulties, etc) so that they're able to access content that would otherwise not be available to them. For example, a description you enter for an image in Keynote can be read out aloud using VoiceOver so that a user with visual impairment is aware of it and understands it.

A text box is totally separate to an image and would need to be added individually. Depending on what you wanted to achieve, you could use a text box, comments or notes. If all you wanted, however, was a caption at the bottom of images then a text box is probably your best bet.