The Upload file screen has three fields.
The Summary field appears below the image on and image page, and allows Wiki markup. Here you should include a brief description of what the image is, its source, and what license you are uploading it under.
Submitting this information will upload the image and save the summary. There is no preview, as there is for editing pages. Nevertheless, after the image is uploaded, you can still click the edit button and make changes to the summary and license, should you make a mistake you did not catch beforehand. If there is already a file uploaded with the Destination filename that you chose, you are given the option to save anyway and overwrite the old file (although it remains in the image history), or returning to the upload form and specifying a new filename.