How to add a background image to a checkbox

I have a checkbox like so:

<input type="checkbox" id="30days" name="checkbox" value="30days" /><label for="30days">30 days</label>

And I tried to add a background image via CSS so the checkboxes look bigger:

input[type=checkbox] { background: url('http://ift.tt/1uAxM1c') no-repeat; margin-right: 10px; }

but the checkboxes are still the same size and it looks like the background image is not applying, what am I doing wrong?