HTML / CSS How to add image icon to input type="button"?

This should do do what you want, assuming your button image is 16 by 16 pixels.

<input type="button" value="Add a new row" class="button-add" />

input.button-add {
    background-image: url(/images/buttons/add.png); /* 16px x 16px */
    background-color: transparent; /* make the button transparent */
    background-repeat: no-repeat;  /* make the background image appear only once */
    background-position: 0px 0px;  /* equivalent to 'top left' */
    border: none;           /* assuming we don't want any borders */
    cursor: pointer;        /* make the cursor like hovering over an <a> element */
    height: 16px;           /* make this the size of your image */
    padding-left: 16px;     /* make text start to the right of the image */
    vertical-align: middle; /* align the text vertically centered */
}

Example button:

Update

If you happen to use Less, this mixin might come in handy.

.icon-button(@icon-url, @icon-size: 16px, @icon-inset: 10px, @border-color: #000, @background-color: transparent) {
    height: @icon-size * 2;
    padding-left: @icon-size + @icon-inset * 2;
    padding-right: @icon-inset;
    border: 1px solid @border-color;
    background: @background-color url(@icon-url) no-repeat @icon-inset center;
    cursor: pointer;
}

input.button-add {
    .icon-button("http://placehold.it/16x16", @background-color: #ff9900);
}

The above compiles into

input.button-add {
  height: 32px;
  padding-left: 36px;
  padding-right: 10px;
  border: 1px solid #000000;
  background: #ff9900 url("http://placehold.it/16x16") no-repeat 10px center;
  cursor: pointer;
}

JSFiddle

If you absolutely must use input, try this:

background-image: url(...);
background-repeat: no-repeat;
background-position: <left|right>;
padding-<left|right>: <width of image>px;

It's usually a little easier to use a button with an img inside:

<button type="submit"><img> Text</button>

However the browser implementations of button for submitting are inconsistent, as well as the fact that all button values are sent when button is used - which kills the "what button clicked" detection in a multi-submit form.