When you use SSL to secure your site (and you should use SSL) you may encounter web browser warnings on some or all of the pages in your site. Some web browsers provide scarier warnings than others, but in general you want to prevent these warnings so that your users are not scared away or concerned by them. This is especially true if you are doing any kind of ecommerce on your site, you certainly don't want your users thinking there is anything insecure about your site or they may be afraid to proceed with a purchase.
Note that the above screen shot is an example from Internet Explorer 8, it has changed in newer versions and will look different than this screen shot in newer versions of Internet Explorer, Chrome or Firefox or other web browsers. The messages will be similar in most browsers.
What Causes the Insecure Content Warnings?
Finding the Cause(s) of the Insecure Content Warning
What you need to look for is:
you need to inspect every place in the page where that occurs and look for anything with http: like src=http:... or src="http:..." or src='http:... they should generally all have either single quotes or double quotes after the equal sign but they don't always and they may not be consistent within a given page.
Fixing the Problem and Avoiding the Problem