I just found this thread, and am giving the idea a fourth thumbs up. Please implement.
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here is the context:
Image tag definitions
Tag Required Description
<image:image> Yes Encloses all information about a single image. Each URL (<loc> tag) can include up to 1,000 <image:image> tags.
<image:loc> Yes The URL of the image.
<image:caption> Optional The caption of the image.
<image:geo_location> Optional The geographic location of the image. For example, <image:geo_location>Limerick, Ireland</image:geo_location>.
<image:title> Optional The title of the image.
<image:license> Optional A URL to the license of the image.
While I think that figuring out how to do the caption text automatically would be difficult, (you could use the alt tag for the image to be the optional image title. Then mabe you could have an additional tab where you could allow the user to hand paste in the optional captions one at a time.....