google image sitemap title and caption, which is more important?

Question

here is the document about google image sitemap, But <image:caption> and <image:title>, which is more important? if i have only one description of this image, which part is better to be put in? I mean put the description in <image:caption>, leave <image:title> is blank, or put the description in <image:title>, leave <image:caption> is blank.

Another question, if I store image in a third part image cloud server(the image domain is not my main site domain), is it matter to make a google image sitemap? such as:

<?xml version="1.0" encoding="UTF-8"?>
 <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
  xmlns:image="http://www.google.com/schemas/sitemap-image/1.1">
 <url>
   <loc>http://www.mydomain.com/somepage.html</loc>
   <image:image>
     <image:loc>http://couldserver.com/image.jpg</image:loc>
   </image:image>
   <image:title>
      a pretty car
   </image:title>
 </url> 
</urlset> 

Answer 1

Those tags are entirely different–and fairly self-explanatory–things. The difference between them isn't really importance, but purpose. Your descriptions will fit best in the image:caption element. Reserve image:title for if/when you have actual titles, such as a named piece of artwork. Some images don't have titles at all, and that's fine; it's why the element is optional. That's not a reason to cram an entirely different kind of data into it.