gm = require( 'gm' );

gm()
    .in( '-page', '+0+0' )
    .in( 'public\media\background.png' ) //Load the background
    .in( '-page', '+150+150' ) //X and Y position of the tree
    .in( 'public\media\tree.png' )
    .mosaic()
    .write( 'result.png', function (err) {

       if ( !err ) console.log( err )

    });