When asking for support, its sometimes beneficial (especially with style related queries) to have a URL. You are free to post it with support requests if necessary or you feel it will help.
Style Properties, Message layout, Like Icon
Upload the image you want and change the url to suit and remove/change the positioning to suit whether it's on a sprite sheet or a single image
Ok, so I uploaded an image I want to use, changed the url, erased the left padding and I get nothing. Just the names of people who like the post, no icon.