let STR = 'src="/sites/default/files/resize/m14/4m213/3567705_images_12175409452-.jpg" src="/sites/default/files/resize/afaf/5814f65f91ef731c793433fe5096e892-.jpg" src="/sites/default/files/resize/test/640m-.jpg" src="/sites/default/files/resize/5173.970-.jpg" src="/sites/default/files/resize/a/b/3/5main_br_xl-.jpg" src="/sites/default/files/resize/images/m8/image8-.jpg"';
let RES = STR.replace(/src=\"([^\"]+\/)([^\/]+\.jpg)\"/g, '/твой_новый_путь_до_картинки/$2');
console.log(RES);
И, если дело реально на "той стороне", то: