According to the docs v4, you are looking for img-thumbnail (which should be used in img tag instead of col-*-* as you have)
Notes:
- In v4
col-xswas replaced bycol- - I’ve added
img-fluidutility 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>