7thcircle on "page was loaded over HTTPS, but requested an insecure image"

when i install and apply SSL certificate on Bluehost webhosting, indeed my site entering secure channel however discovery page request image with http address but not https address even i upload photo to media library through secure channel.

that is bug's of wordpress?

It's not a bug of WordPress. When you went to https that actually was like a server move. You went from http://your-url to https://your-url and the old references need to be changed to the new ones.

Give this a read, it can help you out.

https://codex.wordpress.org/Moving_WordPress#Changing_Your_Domain_Name_and_URLs

Even though the server name hasn't changed, it actually did. The new one is https://your-url

alright, i have a big trouble, because my site is wordpress multi-site, some sub-site without SSL, some sub-site with SSL. have much hard work must to do to solve it.

It depends if you are using subdomains or folders for sites. You can use Better Search Replace https://wordpress.org/plugins/better-search-replace/ to change either folders or subdomains