為什么logo可以，a.png就不可以？

import React, { Component } from 'react';
import './App.css';
import logo from './logo.svg'
import imgs from './a.png'

let imgJson = require('./imageDatas');

function imgURL(imgDateArray) {

    for(var i=0;i<imgDateArray.length;i++){
        var signleImageData = imgDateArray[i];
        signleImageData.imageURL =`./react-card/src/img/${signleImageData.fileName}`;
       // console.log(signleImageData.imageURL)
        return signleImageData.imageURL
        //imgDateArray[i] = signleImageData;
    }
}

var url = imgURL(imgJson);
console.log(url);

class App extends Component {

  render() {
    return (
      <div className="App">
        <header className="App-header">
          <img src={logo} className="App-logo" alt="logo" />
            <img src={imgs} />
          <h1 className=''>Welcome to React  </h1>
        </header>
        <p className="App-intro">
          To get started, edit <code>src/App.js</code> and save to reload.
        </p>
      </div>
    );
  }
}

export default App;