how to put images horizontally in css

a {
    text-decoration: none;
}

img {
    width: 300px;
    height: 300px;
}
body {
    font-family: 'Open Sans', sans-serif;
}

#wrapper {

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

header {
    float: left;
    margin:0 0 10px 0;
    padding: 5px 0 0 0;
}
#logo {
    text-align: center;
    margin: 0;

}

h1 {
    font-family: 'Economica', sans-serif;
    margin: 15px 0px;
    font-size: 18em;
    font-weight: normal;
    line-height: 0.8em;
}
h2 {
    font-family: 'Economica', sans-serif;
    font-size: 1.70em;
    margin: -5px 0 0;
    padding: 15px 50px 30px 10px;
    font-weight: normal;
}

nav {
    font-family: 'Economica', sans-serif;
    font-size: 2.00em;
    margin-top: 80px;
    }


nav ul {
    list-style: none;
    margin: 0 10px;
    padding:0;
}
nav li {
    display:inline-block;
}

nav a {
    font-weight: 800;
    padding: 15px 10px;
    }
a {
    color: white;
}
/*grey header*/
header {
    background: #585858;
    border-color: #yellow;
    width: 1300px;
    height: 450px;

}

h1, h2 {
    color: #fff;
}
nav {
    background: #585858;}



nav a, nav a:visited {
    color: #BDBDBD;
}
nav a.selected, nav a:hover {
    color:white;
    }

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

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

#gallery {
    margin: 0;
    padding: 0;
    list-style; none;
}
#gallery li {
    float: left;
    width: 45%;
    margin: 2.5%;
    background-color: #fff;
    color: #bdc3c7;
    display: inline-block;
}
#gallery li a p {
    margin: 0;
    padding: 5%;
    font-size: 0.75em;
    color: #bdc3c7;

}

body {
    background-color: #fff;
    color: 999;
}