→ Here is the code for small buttons:
.sqs-block-button-element--small{background-color:transparent!important; background-image:url(‘image-url-here'); background-size:contain!important; background-position:center;background-repeat:no-repeat!important}

→ Here is the code for medium buttons:
.sqs-block-button-element--medium{background-color:transparent!important; background-image:url(‘image-url-here'); background-size:contain!important; background-position:center;background-repeat:no-repeat!important}

→ Here is the code for large buttons:
.sqs-block-button-element--large{background-color:transparent!important; background-image:url(‘image-url-here'); background-size:contain!important; background-position:center;background-repeat:no-repeat!important}

→ Add this line of code to make sure the button image won't change opacity on a hover:
Small button
.sqs-block-button-element--small:hover{opacity:1!important; }

Medium button
.sqs-block-button-element--medium:hover{opacity:1!important; }

Large Button 
.sqs-block-button-element--large:hover{opacity:1!important; }

→ Add this line of code before the final bracket to give any PNG image a drop shadow, adjusting the px values and color codes to make it match your site style:
filter:drop-shadow(5px 5px 10px rgba(0,0,0,0.8)