An image element will resize itself to fit the size of the image. Even if it was valid CSS to apply a background-image to an img element, it would always be covered up. Not to mention you can't click a background-image.
A better solution is to write the buttons as HTML. You can use CSS to move the buttons to the far left and far right, or if you want to position them veritcally, top and bottom.
<style>
.image_cont { position: relative; display: inline-block }
/* left and right styling */
.image_next_btn { position: absolute; left: 0; top: 50%; margin-top: -10px; }
.image_prev_btn { position: absolute; right: 0; top: 50%; margin-top: -10px; }
/* or top and bottom styling */
.image_next_btn { position: absolute; left: 50%; top: 0; margin-left: -10px; }
.image_prev_btn { position: absolute; left: 50%; bottom: 0; margin-left: -10px; }
</style>
<div class="image_cont">
<img src="images/Ania_02.jpg">
<div class="image_prev_btn">prev</div>
<div class="image_next_btn">next</div>
</div>