Why are my images not in 2 columns?

body {
  font-family: 'Quattrocento Sans', sans-serif;
}

#wrapper {
  max-width: 940px;
  margin: 0 auto;
  padding: 0 5%;  
}

a {
  text-decoration: none;
}

img {
  max-width: 100%;
}

h3 {
  margin: 0 0 1em 0;
}



/*******************
      HEADING
********************/

header {
  float: left;
  margin: 0 0 30px 0;
  padding 5px 0 0 0;
  width: 100%;
}

#logo {
  text-align: center;
  margin: 0;
}

h1 {
  font-family: 'Merriweather Sans', sans-serif;
  margin: 15px 0;
  font-size: 1.75em;
  font-weight: normal;
  line-height: 0.8em;
}

h2 {
  font-size: 0.75em;
  margin: -5px 0 0;
  font-weight:normal;
}



/*******************
     NAVIGATION
********************/

nav {
  text-align: center;
  padding: 10px 0;
  margin: 20px 0 0;
}

nav ul {
  list-style: none;
  margin: 0 10px;
  padding: 0;
}

nav li {
  display:inline-block;
}

nav a {
  font-weight: 800;
  padding: 15px 10px;
}



/*******************
       FOOTER
********************/

footer {
  font-size: 0.75em;
  text-align: center;
  clear: both;
  padding-top: 50px;
  color: #ccc;
}

.social-icon {
  width: 20px;
  height: 20px;
  margin: 0 5px;
}




/*******************
   PAGE: PORTFOLIO
********************/

#gallery {
  margin: 0;
  padding: 0;
  list-style: none;
}

#gallery li {
  float: left;
  width: 45%
  margin: 2.5%
  background-color: #f5f5f5;
  color: #bdc3c7;
}

#gallery li a p {
  margin: 0;
  padding: 5%;
  font-size: 0.75em;
  color: #bdc3c7;
}



/*******************
     PAGE: ABOUT
********************/

.profile-photo {
  display: block;
  max-width: 150px;
  margin: 0 auto 30px;
  border-radius: 100%;
}



/*******************
   PAGE: CONTACT
********************/

.contact-info {
  list-style: none;
  padding: 0;
  margin: 0;
  font-size: 0.9em;
}

.contact-info a {
  display: block;
  min-height: 20px;
  background-repeat: no-repeat;
  background-size: 20px 20px;
  padding: 0 0 0 30px;
  margin: 0 0 10px;
}

.contact-info li.phone a {
  background-image: url('../img/phone.png');
}

.contact-info li.mail a {
  background-image: url('../img/mail.png');
}

.contact-info li.twitter a {
  background-image: url('../img/twitter.png');
}




/*******************
       COLORS
********************/

/* site body */
body {
  background-color: #fff;
  color: #999;
}

/* green header */
header {
  background: #6ab47b;
  border-color: #599a68;
}

/* nav background on mobile*/
nav {
  background: #599a68;
}

/* logo text */ 
h1, h2 {
  color: #fff;
}

a {
  color: #6ab47b;
}

/* nav link */
nav a, nav a:visited {
  color: #fff;
}

/* selected nav link */
nav a.selected, nav a:hover {
  color: #32673f;
}