根据

According to the docs v4, you are looking for img-thumbnail (which should be used in img tag instead of col-*-* as you have)

注释:

• 在v4中，col-xs被替换为col-
• 我添加了img-fluid实用程序类以使图像具有响应性

• In v4 col-xs was replaced by col-
• I've added img-fluid utility class in order to make images responsive

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">

<div class="container">
  <h2>Pictures of Coffee</h2>
  <div class="row">
    <div class="col-lg-4 col-6 "><img class="img-thumbnail img-fluid" src="https://d2lm6fxwu08ot6.cloudfront.net/img-thumbs/960w/04LDEYRW59.jpg" alt=""></div>
    <div class="col-lg-4 col-6 "><img class="img-thumbnail img-fluid" src="https://d2lm6fxwu08ot6.cloudfront.net/img-thumbs/960w/90V03Q5Y60.jpg" alt=""></div>
    <div class="col-lg-4 col-6 "><img class="img-thumbnail img-fluid" src="https://d2lm6fxwu08ot6.cloudfront.net/img-thumbs/960w/O83SF2RB6D.jpg" alt=""></div>
    <div class="col-lg-4 col-6 "><img class="img-thumbnail img-fluid" src="https://d2lm6fxwu08ot6.cloudfront.net/img-thumbs/960w/5JVPSVP7EI.jpg" alt=""></div>
    <div class="col-lg-4 col-6 "><img class="img-thumbnail img-fluid" src="https://d2lm6fxwu08ot6.cloudfront.net/img-thumbs/960w/C5Y10KIIHA.jpg" alt=""></div>
    <div class="col-lg-4 col-6 "><img class="img-thumbnail img-fluid" src="https://d2lm6fxwu08ot6.cloudfront.net/img-thumbs/960w/YSSFRY5B25.jpg" alt=""></div>
  </div>
</div>