Here you're talking about HTTP requests number -v- Healty SEO
If you care about Search Engines returning your website in the Images Search for that only image i.e. "SWIX" just cause you used a single image with a needed alt
tag, knowing you'll have 21+ requests to the server for your images... than go for it.
If you Want to keep your requests for images to a minimum, than go for sprite.
You can test the difference in any developer tool Network tab - and see the average time that's needed to grab every uncached image from the server.