after installing ssl certificate , browser shows insecure

We have just added ssl certificate on my domain .

Before ssl , we used http:// in whole website, now in general settings > both WordPress Address (URL) and Site Address (URL) I put , but after that all browsers url shows insecure.
Hosting server guys says :

contains insecure items like images
so in the html will be src="http://
like img src="http://url/to/image.jpg
those need to be under ssl as well.

What is the solution for fixing it ? How can we fix it.