Thank you so much for your help. Actually relocation of images from a subfolder to a subdomain was my attempt to appease Google Page Speed, and it indeed increased the Page Speed percentage score from 90% to 95% but frankly, as far as I can tell, the page hasn't sped up from the user-end of things. The page doesn't load faster, just differently. My uneducated guess says that's because using the subdomain as I've done (same shared hosting, same server) just amounts to a 'smoke and mirrors' trick that somehow appeases Google Page Speed. I'm probably way off on that assumption but the bottom line is the page doesn't load any faster when you're sitting at your keyboard, waiting.
You're absolutely right about the sitemap. If I keep the images on the subdomain, it surely can't hurt to have a big red arrow (in the form of a sitemap) telling Google where to find 'em.
With respect to image search: I'm not sure yet exactly how to track conversions made via image search results. The site is heavy on images, though (it's a jewelry site targeted to a fairly specific style of jewelry) and it's my assumption that if someone types in, say, 'butterfly necklace' for example, odds are they're at least needing to browse for a butterfly necklace for whatever reason. And to my way of thinking, 'browsers' means 'potential buyers.' It's not like anyone's going to type in 'butterfly necklace' because they're sitting around, bored. I like vintage Bakelite bracelets, but I don't search for any unless I'm in the market to buy. And so I think indexed images are important. But again, I've been at this a whole six months and a lot of my assumptions have been spot-on wrong.
Thank you again for your assistance, and for your patience with my questions.