I have 6 pictures, my pictures stack with 2 pics next to each other,1 pic alone, 2 pic next to each other, 1 pic alone

ul {
    list-style-type: none;
}

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

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

a {
  text-decoration:none;
}

img {
  max-width: 100%;
}

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

h1 {
  font-family: 'Viga', 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;
}

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

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

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

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


/* link */
a {
  color:#2e8588;
}

/* green header*/
header {
  background: #cdeae5;
  border-color: #2e8588;
}

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

/* nav background on mobile */
nav {
  background: #2e8588;
}

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

/* selected nav link */
nav a.selected, nav a:hover {
  color: #9dd1c8;
}

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