Using a protocol relative URL (e.g. “<img src=”//www.mysite.com/~/media/image.png?mh=50” />) is a great way to load a resource from a different domain over HTTP when the page it is referenced on is on HTTP and over HTTPS when it is on a page which is loaded over HTTPS. You want to ensure Media Request Protection is turned on in case you want to do some scaling of the image as in the sample above to protect your Sitecore instance.

In above scenario it is easy to run into issues as the hash generated to protect the image does not handle the protocol relative url properly. Consider below hash values generated by admin page at /sitecore/admin/mediahash.aspx

1. /~/media/image.png?mh=50 gives hash  07205019C3F1F44438D529F64842DBEE98C0632B
2. http://www.mysite.com/~/media/image.png?mh=50 results in same hash
3. https://www.mysite.com/~/media/image.png?mh=50 also results in same hash
4. //www.mysite.com/~/media/image.png?mh=50 gives a different hash 5A309EF4DC195929BA7EE946A0E66AD6D349712B

I did run in a number of issues because the difference in hash value and decided against using protocol relative URL and instead always load the images over HTTPS which is working well with Media Request Protection. Recent developments also indicate that loading as much as possible over SSL is the better approach see here