This one should work. Unless it is a secured server. In that case HTTPS:// …
Now, the other issue that might cause this problem is the folder’s permission settings. If you use your FTP, Filezilla or anything else, you can view the the permissions set on the folder that the image is in.
Let us know that. I will explain why FTP will not work…
Your browser handles the BROWSER-SIDE code displaying data and images. The server handles SERVER-SIDE programs. You NEVER see the server side items in the browser. Therefore PHP is never in the code when you view-source of a page. This is for security so that hackers can not see your code and hack your site. Now, that is why you can not use FTP the way you have in your HTML. It just is not possible.
Two workarounds are available. Simply change the permissions for the image folder to allow reading of the files OR simply use PHP to insert the image. The first way is the easiest. But, of course it allows hackers to see your images if they know the name of the files. The second way is trickier. You need to create an image object, load it with the base64 version of the image and then send the image object to the browser using headers. Not too many lines to do that, but, more work.
So, the next question is, are you worried about users or hackers seeing the images? If you need them to be 100% secure, do it the second way. If it is okay for them to be seen, the first way is simpler!