My images will not stack as the screen shrinks. What code am I missing?

/*******************************
GENERAL
*******************************/

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

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

a {
  text-decoration: none;
}

img {
  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: 'KoHo', sans-serif;
  font-family: 'Noto 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: .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('../images/phone.png')
}

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

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



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

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

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

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

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

/* links */
a {
color: #6ab47b;
}

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

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