Vertically centering images within flex items

Question

I have four divs sitting inside a parent flexbox.

What is confusing me is how I can get the images within the flex items to center vertically and horizontally. I thought the following would do it:

justify-content: center; align-items: center;

Adding these to the flexbox parent would work but it doesnt seem to have the effect I am looking for.

My CSS is as follows:

 .flex-container {
   display: flex;
   flex-wrap: nowrap;
   width: 650px;
 }

 .flex-container > div {
   width: 100px;
   margin: 10px;
   text-align: center;
   line-height: 75px;
   font-size: 30px;
   align-items: center;
   justify-content: center;
 }

 .div-1 {
   flex: 2;
 }

 .div-2 {
   flex: 1;
 }

 .div-3 {
   flex: 1;
 }

 .div-4 {
   flex: 1;
 }

 .img {
   height: 55px;
 }

 .bookit_button {
   background-color: #4CAF50;
   border: none;
   color: white;
   padding: 15px 32px;
   text-align: center;
   text-decoration: none;
   display: inline-block;
   font-size: 16px;
   margin: 4px 2px;
   cursor: pointer;
 }

My HTML is as follows:

   <div class="flex-container">
     <div class="div-1">
       <button class="bookit_button">Book An Appointment</button>
     </div>
     <div class="div-2"><img src="images/facebook_icon.png" class="img"/></div>
     <div class="div-3"><img src="images/instagram_icon.png" class="img" /></div>
     <div class="div-4"><img src="images/twitter_icon.png" class="img"  /></div>

Ultimately, the goal is to end up with this:

http://thelavender.net/_fades/images/sample.png