Instead of copying the image to the server, just upload it through QMC in Content Libraries.
By just copying the image to disk there is no record of the image in the repository database.
Also depending how your local folder structure was set up you might have to re-add the image once the app is uploaded to the server and before you publish it.
Yes, that is the correct process. Make sure you uploaded the file to the Default library otherwise it won't show up in the client, that will be enhanced it future versions.
In QMC, can you see your image listed in the library? Enable Contents in the right hand menu as per the screenshot. If you see your image, great then it's uploaded correctly. Try to browse to it by clicking the blue hyperlink next to it, if you can see it then most likely there are no security issues.
Have you changed any of the default security rules that the product ships with? If so you can audit that you still have access to the images by clicked on Users in the right hand menu and see that you are listed with Read access.