You can right click on the border using the paint palette and select T (on the right top corner of the paint selection palette). This will make your border transparent.
You need to bring up your Tools palette and select the brush tool at the bottom. Now right click on the boarder of the picture control and in the color palette, select the small box in the upper right with the little 'T' in it. This is the transparent color.
The attached image shows the same picture with and without a boarder.