Or if you're techie...
Add the following code (in bold) to your blog sidebar:
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<title>Just a Card Blog Button HTML</title>
</head>
<body>
<div align="center"><a target="_blank"
href="http://www.justacard.org/"><img alt="Just a Card Button"
src="https://static.squarespace.com/static/503b5414e4b04953d0f3fdee/t/5471dd61e4b04536cee1a54c/1416748385728/?format=300w&storage=local"
border="0" height="150" width="150"></a></div>
</body>
</html>