I think you can assign a specific jpg per folder via the standard Properties dialog (in Opus or File Explorer)... But it's going to create a (hidden) desktop.ini file in the folder which isn't really better than just copying the jpg, and is a lot more time consuming since you have to do it for each folder individually.
On the "choose a file to show..." option I think the default value is folder.jpg but you can pick another image with any filename you want without the need for dektop.ini file.
That image would still need need to be inside the folder but you could name it the way you want at least.
Not to be confused with the "change icon option underneath" I edited out the picture. That one would indeed create a desktop.ini file inside the folder. But as Leo mentioned it is also the only way to have a fully maximized icon for non-music folders.
For others you would have to set flat folder view in dopus for the folder image to look as good as possible with minimal borders.
If the image is the same size ratio as the flat folder then it will fill it completely.
Here is an example with music albums. Those are the only ones that don't need flat folder view if I recall. For movies and other best to use that.