In Jnes 1.0 there is an artwork rom browser mode where you can have view the list of games as boxart for example. Our very own Gent put together a boxart pack for US region games and it is provided as a download below. Extract these files where you installed Jnes in a subfolder called "artwork"

One important note is in order to use this pack your roms have to follow the Good Rom Names convention! Wiki

 Download pack USA Complete, by Gent!