You can use HTML and some JavaScript. This will be simple code, if you're looking for the code to be highlighted then that's something a bit more complex:
HTML:
<button id="thebutton >Image</button >
<textarea id="thetextarea" ></textarea >
JavaScript:
$('thebutton').click(function(){
$('#thetextarea').append('[img][/img]');
});
HTML:
<button id="thebutton >Image</button >
<textarea id="thetextarea" ></textarea >
JavaScript:
$('thebutton').click(function(){
$('#thetextarea').append('[img][/img]');
});