Here is a site to compare your duck eggs too: http://www.metzerfarms.com/Candling.cfm
I'm also incubating (call duck) eggs at the time and they're on day 17. When I was on day 7 you could see vein-like "spiders" in the most of the eggs, but some looked like yours and they're growing now, so I say wait several days and candle again. Good luck!