If you uploaded the photo using the uploader tool, it should appear on the button with small boxes on the corner of the photo.  If you drag a corner box out, it should make the photo bigger.  If you pull it in, it should make the photo smaller.

If the picture uploaded takes up the entire face of the button, you may have to drag the picture off center to make the corners appear.

There are some browser extensions that work adversely with the design tool.  If for some reason dragging the boxes does not make the photo bigger or smaller, you may have to try the design tool in a different browser.