margin: auto is not centering

Define width or margin on your #sponsors ID

as like this

#sponsors{
margin:0 auto; // left margin is auto, right margin is auto , top and bottom margin is 0 set
width:1000px; // define your width according to your design 
}

More about margin auto

You must specify width to div and don't give margin twice

#sponsors {
    margin:50px auto 0 auto;
    margin-top:50px;
    overflow: hidden;
    width:160px;
    background:aqua
 }

DEMO

For centering DIV you need to set css for below.

Example

#sponsors {
   margin:0px auto;
}

Comment

You also need to set width for div.

DEMO

No need of using margin: 0 auto. Try the below code, it will work:

div#sponsors{
    /* other css properties */ 
    /* remove display:inline-block and margin: auto */       
    width:100%;  /* instead of width: auto */
    text-align: center;

}

div.img{
    /*remove float:left */
    /* other css properties */
    display: inline-block;
}

Remove text-align: center from body tag and give to h1 tag instead.