Pure semantic HTML/CSS solution

This is easy to implement on your own, no pre-made solution necessary. Also it will teach you a lot as you don’t seem too easy with CSS.

This is what you need to do:

Your checkboxes need to have distinct id attributes. This allows you to connect a <label> to it, using the label’s for-attribute.

Example:

<input type="checkbox" id="myCheckbox1" />
<label for="myCheckbox1"><img src="http://someurl" /></label>

Attaching the label to the checkbox will trigger a browser behaviour: Whenever someone clicks the label (or the image inside it), the checkbox will be toggled.

Next, you hide the checkbox by applying for example display: none; to it.

Now all that is left to do is set the style you want for your label::before pseudo element (which will be used as the visual checkbox replacement elements):

label::before {
    background-image: url(../path/to/unchecked.png);
}

In a last tricky step, you make use of CSS’ :checked pseudo selector to change the image when the checkbox is checked:

:checked + label::before {
    background-image: url(../path/to/checked.png);
}

The + (adjacent sibling selector) makes sure you only change labels that directly follow the hidden checkbox in the markup.

You can optimize that by putting both images in a spritemap and only applying a change in background-position instead of swapping the image.

Of course you need to position the label correctly and apply display: block; and set correct width and height.

Edit:

The codepen example and snippet, which I created after these instructions, use the same technique, but instead of using images for the checkboxes, the checkbox replacements are done purely with CSS, creating a ::before on the label that, once checked, has content: "✓";. Add some rounded borders and sweet transitions and the result is really likable!

Here is a working codepen that showcases the technique and doesn’t require images for the checkbox:

http://codepen.io/anon/pen/wadwpx

Here is the same code in a snippet:

ul {
  list-style-type: none;
}

li {
  display: inline-block;
}

input[type="checkbox"][id^="cb"] {
  display: none;
}

label {
  border: 1px solid #fff;
  padding: 10px;
  display: block;
  position: relative;
  margin: 10px;
  cursor: pointer;
  -webkit-touch-callout: none;
  -webkit-user-select: none;
  -khtml-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

label::before {
  background-color: white;
  color: white;
  content: " ";
  display: block;
  border-radius: 50%;
  border: 1px solid grey;
  position: absolute;
  top: -5px;
  left: -5px;
  width: 25px;
  height: 25px;
  text-align: center;
  line-height: 28px;
  transition-duration: 0.4s;
  transform: scale(0);
}

label img {
  height: 100px;
  width: 100px;
  transition-duration: 0.2s;
  transform-origin: 50% 50%;
}

:checked+label {
  border-color: #ddd;
}

:checked+label::before {
  content: "✓";
  background-color: grey;
  transform: scale(1);
}

:checked+label img {
  transform: scale(0.9);
  box-shadow: 0 0 5px #333;
  z-index: -1;
}

<ul>
  <li><input type="checkbox" id="cb1" />
    <label for="cb1"><img src="https://picsum.photos/seed/1/100" /></label>
  </li>
  <li><input type="checkbox" id="cb2" />
    <label for="cb2"><img src="https://picsum.photos/seed/2/100" /></label>
  </li>
  <li><input type="checkbox" id="cb3" />
    <label for="cb3"><img src="https://picsum.photos/seed/3/100" /></label>
  </li>
  <li><input type="checkbox" id="cb4" />
    <label for="cb4"><img src="https://picsum.photos/seed/4/100" /></label>
  </li>
</ul>