K2Store will display the Add to cart as a button by default. If you want to use the image, you can do so by using CSS.
First, upload the image file to your template's images directory.
Then add the following CSS at the end of your template's css file.
NOTE: Make sure that the image's extension type is entered correct. If you are using a JPG format of image file, then you the image file should read as your_image_file_name.jpg