Not sure why the question has been downvoted, I had a similar question and this helped, but the other answer didn't work for HTTPS.
For HTTPS:
Night time image:
https://s.yimg.com/zz/combo?a/i/us/nws/weather/gr/28n.png
Day time image:
https://s.yimg.com/zz/combo?a/i/us/nws/weather/gr/28d.png
You will see that the last number (28 in this example) is the condition code from the Yahoo API Response plus a code to represent day 'd' or night 'n'.
I noticed the images are quite different, firstly they are PNG's but they are also larger and include transparency.
Jim H's answer is correct for HTTP.