You absolutely need a button tag element? because you can use instead an input type="button" element.
Then just link this CSS:
input[type="button"]{
width:150px;
height:150px;
/*just this*/ background-image: url(https://images.freeimages.com/images/large-previews/48d/marguerite-1372118.jpg);
background-position: center;
background-repeat: no-repeat;
background-size: 150px 150px;
}
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<input type="button"/>
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