The purpose of the owp_ prefix is an internal use thing by the plugin. It was a way to allow the plugin to know to retrieve the image from the plugin zip file, rather than look for it in the resources directory. If you add an image with that owp_ prefix, the plugin will be looking for it in the zip, hence why it doesn't work whereas renaming and removing the prefix does.
One day I may look to implement it another way, but to be honest it never occurred to me that people would copy my prefix convention, that was why I made it fairly obscure
