<input type="image" src="img/beer.png" /> is for collecting coordinates. If you want to use it as a submit button, you need to add onsubmit -event, for example.
<input type="image" src="img/beer.png" onsubmit="submit();" />
But it is better to use the <button> element, which is more flexible. It can contain text, images, or both:
<button type="submit" name="beer" value="beer_btn_was_clicked"> Here some optinal text <p> you can even put it in a paragraph! </p> And you don't even need JavaScript! <img src="img/beer.png" /> </button>
Edit (2016-02-12)
To date, * the above example is not considered 100% valid, since <p> elements are no longer allowed in a <button> element .
According to the link to the MDN HTML element, the permitted content category in the <button> element is the so-called Content Frasing :
The content of the phrase defines the text and the inscription contained in it. Phrasing entries make up paragraphs.
Elements belonging to this category are <abbr> , <audio> , <b> , <bdo> , <br> , <button> , <canvas> , <cite> , <code> , <command> , <datalist> , <dfn> , <em> , <embed> , <i> , <iframe> , <img> , <input> , <kbd> , <keygen> , <label> , <mark> , <math> , <meter> , <noscript> , <object> , <output> , <progress> , <q> , <ruby> , <samp> , <script> , <select> , <small> , <span> , <strong> , <sub> , <sup> , <svg> , <textarea> , <time> , <var> , <video> , <wbr> and plain text (consisting of more than just space characters).
Several other elements fall into this category, but only if a specific condition is met:
<a> if it contains only phrasal content<area> if it is a descendant of an element<del> if it contains only phrasal content<ins> if it contains only phrase text<link> if itemprop attribute is present<map> if it contains only phrase text<meta> if itemprop attribute is present
* today was what I read about it, and not when the change was made