Good morning.
I wanted to know if it was possible to create a sitemap images starting from the primary domain www.mysite.it.
The images are on a third-level domain example: media.mysite.it
Thanks 

Hi,

Sure I just updated this help page:
http://www.microsystools.com/products/sitemap-generator/help/image-xml-sitemaps/

Essentially, just use the "quick scan" preset:
Quick presets | Google image sitemap - website has images hosted externally

thank you very much